As far as we know, the Universe we live in is infinite, but we still know that it began about 13.8 billion years ago in the Big Bang. Since then, the Universe has been expanding outwards in every direction, and recently, the expansion of the Universe has started to speed up due to a mysterious force called “dark energy.” In addition, the evolution of the Universe shows clear evidence of an unseen substance called “dark matter” that weighs several times more than all of the “normal matter” in the Universe but is completely unseen.

The nature of the Universe and its expansion are hard to wrap your head around, requiring a good understanding of some complex physics, astronomy, and general relativity to fully comprehend. We do our best to answer these questions below.


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The Big Bang

Are the various millions or billions of galaxies the result of “Big Bangs” separate and apart from our own ”Big Bang”? (Beginner)
The Big Bang was a lot “bigger” than you are thinking of. Not only were all of the galaxies in the Universe created in one Big Bang, everything in the Universe was created in the Big Bang. That includes all of the gas, light, and dark matter that exists between galaxies. It also includes the abstract concepts of space and time, which, to our knowledge, don’t exist in the same way outside of our Universe. The Big Bang is responsible for everything in the Universe.
However, the current Universe didn’t spring fully formed from the Big Bang. Originally, everything in the Universe was just an infinitely compressed ball of energy, and as the Universe expanded, this energy “condensed” down into particles and atoms that eventually went on to form the elements we know today. But these elements weren’t in the form of galaxies, they were a cloud of gas that was basically uniform across the entire Universe. Slowly over time, though, gravity started pulling everything together into larger and larger clumps, eventually forming stars, galaxies, and galaxy clusters. Here is a computer simulation of what astronomers think happened after the Big Bang. Each one of the small clumps is a galaxy, and they slowly condense into larger and larger clumps that represent giant galaxies and galaxy clusters.
For the singularity at the beginning of the Big Bang, what was its size, composition, and how would it compare to other subatomic sized objects in the universe? (Beginner)
If I could answer your questions about the singularity at the beginning of the Universe with any sort of detail, then I should just hop on a plane to Stockholm to go collect my Nobel Prize! Currently, there is basically nothing known for sure about the beginning of the Universe. We know that the Universe is currently getting bigger, so if we go back in time, we would expect it to get smaller, and by knowing the rate, we can predict when it would have started from zero.
The “size” of the singularity is difficult to define because we usually measure something relative to something else, but we’re talking about the entire Universe so there is no “something else” to measure it with. The only real answer I can give to that it still contained all of the “stuff” that was in the Universe today but squeezed into an infinitely small space. Comparing anything to “infinitely small” is going to make the other thing look big, so it doesn’t matter whether we’re comparing it to an electron or the Earth, it’ll still be infinitely small.

Expansion of the Universe

How does dark energy contribute to the expansion of the Universe? (Beginner)

Dark energy is a hypothetical property of the Universe proposed in order to explain the accelerating expansion of the Universe we observe. In the late 1990’s, astronomers calculated the rate of expansion of the Universe by observing supernovas (exploding stars) across large distances in the Universe. When they plotted how fast the supernovas were expanding away from us as a function of their distance, they found that although the Universe has been expanding for its entire history, it has been expanding faster recently, meaning that something must be making the expansion speed up.

The currently accepted model of this is that empty space itself must have some amount of “dark energy” contained within it driving the expansion. If you picture a box filled with nothing but empty space, this energy will push outwards on the walls of the box and make it want to expand. Expanding the box will mean that there is more empty space inside the box, so there will be more pressure pushing outwards, so the box expands faster. This process can be seen in the expansion history of the Universe and will eventually lead to runaway expansion as the energy from empty space overwhelms the Universe.

Dark energy has never actually been observed in a lab (hence the “dark”) but we see very good agreement with this model in things like the cosmic microwave background and the arrangement of galaxies in the Universe. It also fits well into Einstein’s theory of relativity, which is our current theory for how space, time, and gravity work. So overall, dark energy is responsible for increasing the expansion rate of the Universe and will become increasingly dominant as time goes on.

What caused the special state of matter that became repulsive and triggered the Big Bang? (Intermediate)
Very little is known for sure about the beginning of the Universe. We have theories based on what we observe the Universe to be like now, but proving those theories (especially their predictions about what happened before the Big Bang) is essentially impossible. One thing we are sure of is that the Universe is expanding and it has been for its entire life. Different things in the Universe affect the expansion of the Universe in different ways (matter pulls it in with gravity but dark energy pushes it outwards). We can track what stuff has been in the Universe over its whole lifetime by seeing how it has been expanding, a technique that gets us back pretty far, but not all the way to the Big Bang.
One theory that sounds a lot like what you are explaining here is called inflation, which says that in the very early Universe (around 10^-35 seconds after the Big Bang), the Universe increased its rate of expansion immensely, making it get bigger by a factor of 10^26 by in the next 10^-32 seconds. The proposed reason for this is difficult to explain, but essentially it is proposed that there is a field called the inflaton field that controls the expansion of the Universe, and during this short period, the field got bumped into a higher energy state and caused the expansion to speed up significantly before it fell out of that state again shortly after.
Inflation solves a few problems that are raised by our current model of the Universe. For example, we expect the Universe to be curved similar to the way the surface of a balloon is curved. However, when we try to measure the curvature of the Universe, we measure it to be exactly flat. Inflation is like hooking up a pump to that balloon that blows it up at a constant rate but then turning it on really really high for a few seconds to blow the balloon up really fast to suddenly make the balloon gigantic (if we want to keep the scales consistent, inflation would be like making the balloon go from about a foot across to almost the size of the Universe itself basically instantaneously). So even though the balloon is still technically round, it’s so big that it seems flat to people on the surface.
Another problem that this solves is that different parts of the Universe look very similar to each other despite it being impossible for them to ever communicate with each other. We can look at one side of the sky and it will be exactly the same temperature as the opposite side of the sky despite them being so far apart that you wouldn’t be able to get from one part to another even if you had been travelling at the speed of light for the entire age of the Universe. However, if these two parts of the sky had synchronized their temperatures before inflation happened (ie when the balloon was still only a foot across) then they could look similar even when they get moved really far apart from each other.
So inflation basically says that for a very short amount of time at the beginning of the Universe, there was an extremely powerful force that expanded the Universe to be incomprehensibly large in a tiny amount of time and then it stopped and let the Universe continue on its way. This theory fits the observations we have very well, but it’s very difficult to prove conclusively since it’s hard to know things for sure about things that happened so soon after the Universe started.
Is the expansion of the Universe better described by matter moving away from itself in space or by space itself expanding? (Intermediate)

Spacetime itself is expanding (‘stretching’) and as a result, galaxies are moving away from each other. However, this “Hubble expansion” also causes the mass density of the Universe to decrease with time because the same constant mass is being spread out over a larger volume (so at least part of your first explanation is also correct).

One implication of this decreasing mass density is that we don’t completely understand how dark matter halos (the objects within which galaxies live) grow their mass and radius over time. Dark matter halos are defined as “spherical overdensities” in the matter density field, and their radius is defined so that the density within the sphere is some number (usually 200) times the mean density of the universe. But if the mean density of the universe is decreasing with time, then you would need a larger radius to enclose the same overdensity at later times. So did a halo’s radius/mass grow because it genuinely accreted more mass, or because the mean mass density of the universe decreased? This is called dark matter halo pseudoevolution and is an active area of research — related to your question about the internal gravitational binding energy of objects overcoming the cosmic expansion.

Finally, and perhaps counter-intuitively: The expansion is accelerating and is thought to be due to dark energy, whose energy density stays constant with time. This implies that there is more dark energy now than in the past (otherwise the dark energy density would decrease, since the volume keeps increasing due to the expansion).

If the universe is infinite, how did it come from a single molecule? If it can fit in something, wouldn't that mean it has edges? Also since the universe is infinite, it doesn't have a center. Therefore, how is everything moving away from us? if everything is moving outward wouldn't that mean that we are the center? (Intermediate)

These are all great questions that definitely confuse a lot of people when they learn about cosmology, and they are all related to the fact that the Universe is a 3 dimensional bubble in a higher dimensional space. This is a weird concept, so a useful analogy is to think of the your place in the Universe as like an ant on a balloon. To the ant, the surface of the balloon looks like it is just flat ground. It would think that the world it is inhabiting is just a flat two-dimensional space since it can move in the X direction and the Y direction, but it is not aware of the Z direction going in/out from the balloon’s center. So even though this balloon exists in 3D space, the ant’s world is 2D. 

If we inflate the balloon, then every part of the balloon stretches out uniformly, meaning that any two points on the surface of the balloon will get further away from each other. I put a picture demonstrating this below:


The ant staying in one place will think that the ground it’s staying on is stationary and that everything else is moving away from it, but really the ant is being carried outwards just the same as everything else. If you have a balloon lying around, I recommend drawing dots on the surface and blowing it up to see this in action.

For our Universe, pretty much the same thing is happening, with us being the the ants on the surface of the balloon. However, instead of the surface of the balloon being 2D, the Universe is  3D and the balloon/Universe is being blown up in higher dimensional space (the currently leading theory says 11D). So let’s use this analogy to look at the answers to your questions.

How is everything moving away from us? This is the same as for the balloon. As the Universe gets “blown up,” the space between any two points in empty space will expand, so even though we are on expanding space, we will think that we are stationary and everyone else is expanding away from us, which is indeed what we observe.

Where is the center? The center of the balloon isn’t on the surface of the balloon, it’s somewhere off in the “higher dimensional” 3D space. The same goes for the Universe, where the center of our expansion is in some 11D location that we can’t comprehend or observe.

How could an infinite Universe fit in a small space? When we talk about the Universe fitting into a small space, we have to thin about it in the higher dimensional space rather than from our perspective. Think about folding a piece of paper in half a bunch of times. You’re taking a large 2D object and compressing it down into a smaller 3D space. If the sheet of paper was infinitely thin, you would be able to just keep folding it in half forever, compressing the 2D object infinitely. So any object that has fewer dimensions than the space it inhabits can be compressed to take up effectively no space. Our Universe started at a single small point in this 11D space and has been expanding in a 3D way since then.

Does the Universe have edges? This is more of a question of semantics than anything else. Current theories say that the Universe is flat and infinitely large (like the piece of paper I was just talking about), so it has no edges (this is also kind of like the surface of the balloon also doesn’t have edges, although we have never observed the Universe to be curved like the balloon’s surface). However, we can’t see the entire Universe because the Universe is only 13.8 billion years old and light can only travel so fast. So the furthest things we can see are things that gave off light at the beginning of the Universe and that have been sending their light our way for that entire time, and we can’t see anything further away than 13.8 billion light years (the distance light travels in a year). So in this sense, the area of the Universe we can actually see (the “observable Universe”) does have edges and those edges are 13.8 billion light years away from us.

What is the Universe expanding into? (Intermediate)
ltimately, there isn’t really a perfect metaphor I can come up with because the idea of “not space” isn’t something that we, as three dimensional beings, can really understand. However, I will still try to explain this in a way that may still be confusing but will be at least pretty close to the truth.
The classic metaphor for explaining the expansion of the Universe is being an ant on the surface of a big balloon. As far as the ant is concerned, its world is 2D, consisting of just the surface of the balloon and nothing else. The balloon is big enough compared to the ant that it can’t really tell that the balloon is curved (just like we can’t really tell that the Earth is round in everyday life). However, if there were 2 ants that were 1 inch away from each other and then someone blew up the balloon to be twice as big, then those ants would then be 2 inches away from each other (see picture below). The ants could try to find the center of the expansion, but no matter where they looked in their 2D surface of the balloon, they would find the surface expanding uniformly. New balloon surface is constantly being created, but it isn’t being created from anywhere in particular or displacing anything else, it’s just expanding.
The problem here is that since the balloon is a sphere and the ants just live on its surface, the balloon exists in a higher dimensional 3D space than the 2D space the ants exist in. When the balloon expands, it takes up more 3D space to create more 2D space, an expansion that the ants can’t ever really see the true nature of. So this is what is basically happening with our Universe. Space is expanding like the surface of the balloon, but it isn’t taking up any “empty space” to do so in our 3D world, Instead, it seems like it is expanding into a higher dimensional space around us. There are still a few caveats to talk about:
  1. As far as we can tell, the Universe is not a sphere like the balloon in the analogy. All of our (increasingly precise) measurements say that it is perfectly flat, so the expansion isn’t totally like a balloon.
  2. We don’t know anything really about what higher dimensional space the Universe could be living in. There are theories and observations that could confirm these theories, but at present we know nothing.
I don’t think it’s really possible to truly understand it since our brains can’t think in anything other than 3D, but this is basically how I think of it.
There was a period early on during the big bang when the matter and energy in the expanding universe was travelling at many times the speed of light. What happened to time during this period? (Intermediate)

I think the period of the Universe’s expansion you’re talking about is inflation, which was an incredibly short period of incredibly fast expansion almost immediately after the Big Bang. It’s kind of hard to talk about what happened to time during that period though for a few reasons. First, it only lasted for an almost unfathomably short amount of time (~0.01 nonillionths of a second) so it’s not like things could really move or change much over the course of it. Furthermore, the early Universe didn’t even really have that much stuff in it that could really experience time in the way we think about it. There were no molecules or atoms or even protons or quarks. It was essentially just a soup of energy and quantum probability. But I think the core of your question is how the rate of time flow would be affected if space was being stretched so dramatically, and the answer to that is that we don’t really know or have the capacity to know. We experience time at a rate of 1 second per second and that is what controls the rate of chemical reactions in our brains, so there isn’t really any way to talk about time flowing at different speeds because we (and the rest of the Universe) would experience it the same way regardless.

Could the acceleration of the expansion of the Universe be caused by a shell of matter in some super-universe outside the Universe pulling the Universe with its gravity? (Advanced)
The short answer to your question from a cosmological perspective is that the acceleration in the expansion of the Universe that we ascribe to dark energy doesn’t look like “negative gravity” but instead looks like something completely different, and a shell of matter outside of the Universe wouldn’t actually pull the Universe outwards in the way you would expect.
The equations we use to describe the geometry of the Universe are called the Friedmann equations and they have their roots in general relativity. Each type of substance in the Universe (photons, neutrinos, matter, dark matter, dark energy) interacts with these equations according to its equation of state, which is a measure of how that substance changes as the Universe expands. For normal matter, the equation of state is easy to figure: as the Universe expands, the matter stays the same. However, for dark energy, we observe that the more the Universe expands, the more dark energy there is. The specific nature of this, which we can learn about from the Cosmic Microwave Background, has led cosmologists to theorize that dark energy is a property of space itself. This constant creation of more dark energy is almost like a virus, meaning it leads to the Universe expanding exponentially over time. If it was instead some “negative gravity” that was propelling the acceleration, we would expect it to look parabolic, like something falling to the ground, or maybe something different (my theoretical cosmology is rusty). But gravity should never produce an exponential expansion.
In addition, a uniform shell of matter outside of the Universe wouldn’t actually cause the Universe to expand. It can be shown in numerous ways that the gravity inside of a shell of matter is always 0 because the gravitational pull of all of the different parts of the shell will perfectly cancel out. This is a pretty astonishing result, and it means that a shell in a super universe wouldn’t end up having any effect on our Universe.
The only way it could start having an effect is if the shell were not uniform, in which case the gravitational forces wouldn’t cancel out. This, however, would cause certain parts of the Universe to be pulled differently than other parts of the Universe, which goes against the observations that we have made that show a perfectly homogeneous Universe.
Hopefully this all makes some amount of sense, and sorry to burst your bubble. Astronomers are very eager to explain away dark energy with some already-known piece of physics since nobody really has any idea what it is, so I can assure you that if something like this could work, people would be all over it. Of course, it’s always worth coming up with new ideas since we really don’t know what could exist outside our Universe, so maybe the answer to dark energy in fact lies out there.
The Wikipedia page on comoving cosmological distance to the edge of the observable Universe is 13.8 billion light years, and that that distance has stayed the same for the whole life of the Universe. Why has it stayed constant even as the Universe has expanded? (Advanced)
This is a very astute observation that gets into the complex world of cosmological distance measures that often trip people up (including me in an early draft of a recent paper I submitted to a journal). When measuring distance over very large scales in the Universe, we have to be very specific about what we mean by “distance”. Imagine that you have an infinitely long tape measure and you want to measure the distance between two galaxies. You can leave one end of the tape measure floating in space in one galaxy, hop in a spaceship to fly from one galaxy to the other, and then read off the distance on your tape measure to high precision. However, what you have not accounted for is that during your journey, the distance between the two galaxies has grown due to the expansion of the Universe, so the distance you measure now is different from what you would have measured if your spaceship was infinitely fast. Also, the end of your tape measure will not even be in the same place in the original galaxy now since the galaxy has expanded away from where you currently are. Long story short, this way of measuring distance has a lot of potential complications and can give some weird answers.
So in order to measure distances consistently in the Universe, astronomers have defined a few different ways of measuring distance that tell slightly different stories. Comoving distance is one of those, and it is essentially the same as drawing a grid on a balloon. If we draw the grid on the balloon when it is half blown up, then the grid will shrink if we let air out or grow if we inflate it more, but the number of grid lines will always be the same. This grid moves along with the surface of the balloon as it expands or contracts, so we call it “comoving”. This is useful because it can give us a measurement of the amount of space between two grid points that doesn’t change with time. If we were to use a tape measure to get the distance instead (the proper distance), then the distance between one point and another would depend on how much the balloon has been blown up.
In the Universe, we have defined the comoving coordinate system based on how big the Universe looks to us right now, but if we rewind/fast forward time using a computer simulation, things will stay in the same places on the comoving coordinate grid even as the Universe expands or contracts. There may be cases when proper distance is more useful, but cosmological simulations tend to use the same comoving grid size in order to stay consistent. Other distance measures exist too, like redshift (the amount that light has been stretched out by the expansion of the Universe on its journey from one place to another), the angular diameter distance (how far away you would guess something would be based on how large it appears to be in the sky), or the luminosity distance (how far away you would guess something would be based on how bright it appears to be). Each of these has its own specific use cases and peculiarities that you can read about here if you like (they get very mathy though, so be warned).

The Observable Universe

Are there still some galaxies moving away from us and we may never see their light? (Beginner)

This is indeed what we believe to be true in the Universe. Astronomers currently have a lot of evidence that points to the Universe being infinite in size, but we can only see 13.8 billion light years in any direction, a relatively small bubble of the overall Universe. This means that if a galaxy started too far away from us (say, 50 billion light years), its light wouldn’t have enough time in the 13.8 billion year life of the Universe to get to us by now, so we wouldn’t be able to see it. As the Universe gets older, it becomes possible to see more and more galaxies since their light has had more time to traverse the Universe, but this is complicated by the fact that the expansion of the Universe is carrying them ever further away from us all the time. So there are galaxies that will always be effectively impossible to see because they are so far away that even by the time we see them, they will be so far away they will be essentially invisible.

Where is the flash of light from the annihilation of the matter and antimatter at the beginning of the Big Bang? (Intermediate)

Annihilation would have happened very early in the Universe (around 1 second after the Big Bang), releasing a huge number of photons as you said. However, the problem is that the Universe didn’t turn transparent until more than 300,000 years after the Big Bang, so these photons would just bounce around, spreading out the energy into a diffuse glow. Eventually, when the Universe did turn transparent (when atoms finished “condensing” out of the cloud) then the photons would be free, but any distinct flash would be long since smeared out by then. Instead, we see what astronomers call “the surface of last scattering” (the Cosmic Microwave Background) which is when the photons finally broke free of the clouds of matter in the early Universe.

If we sent a space probe far away from Earth, would it be able to see things that are beyond the edge of our observable Universe? (Intermediate)
As of today, any observer can see for about 13.8 billion light years in any direction in a sphere centered exactly on them. An observer 1 light year away at the same time would also see a 13.8 billion light year sphere, so you are correct that that observer would be able to see 1 light year further away in that direction.
However the added nuance that needs to be considered here is that the size of the observable Universe is always expanding at the speed of light. The way that we define “the edge of the observable Universe” is the furthest away place that has had time to send its light to us. Every year the Universe continues to live, there is more time for light to travel, so the size of the observable Universe expands. This means that in 1 billion years, the size of the observable Universe will be 14.8 billion light years.
This presents a problem with your idea. Let’s say that we launch a space probe at 10% the speed of light (way faster than we can launch things today) to set up a telescope on a planet that is 10 light years away. The probe will arrive at the planet 100 years from now, set up its telescope, and look off in the direction where it can see 10 light years further than the Earth can. The problem is that in the 100 years it took for the probe to get there, so the Earth can now see the stuff that was 10 light years out of its reach when the probe was launched, so it’s not new anymore. And if the probe tried to relay the “new” information it can see now back to Earth, it could only do so at the speed of light, meaning the probe’s information would arrive at Earth at the same time as the “new” light itself. The probe would have to be able to travel or relay information faster than the speed of light in order to give the Earth any information that it couldn’t already see itself.
So we have stumbled upon the idea that the “edge of the Universe” isn’t really just the edge of where light can reach us, it is also the edge of where causality can reach us. No information can travel faster than light, so no information from outside that bubble can ever reach us (without breaking our current understanding of the laws of physics of course). So while it would be cool to have an observatory on another planet for a lot of reasons, seeing outside the observable Universe isn’t one of them.

The Properties of the Universe

Why is the Universe flat and not spherical? (Beginner)

There is no particular reason why the Universe should be flat. In fact, this seeming coincidence is a major problem that any modern theories of cosmology have to account for. The prevailing hypothesis for how the Universe got flat (and other otherwise unexplained phenomena) is called Cosmological Inflation, which basically states that almost immediately after the Universe began, it expanded so fast that it smoothed out any curvature that might have existed, making it look perfectly flat. This is essentially the same as blowing up a balloon to be so big that the ant wouldn’t be able to see that it is curved.

Does the Universe look different if we look in different directions? (Beginner)

One of the key properties of the Universe is that it is uniform on large scales. Everywhere in the Universe had essentially the same starting conditions after the Big Bang, so everywhere should have developed exactly the same as everywhere else once you average out local disturbances like galaxy clusters. This can be seen in maps of the Universe that we have made so far, where there may be small features like galaxies and clusters that deviate from average, but on the whole, each part of the picture looks the same. So since the Universe looks the same on average no matter where you are, it shouldn’t matter which direction JWST is pointing when it looks to the beginning of the Universe. The first stars and galaxies should be approximately the same no matter where you look. The flatness of the Universe doesn’t really come into this directly, that really just dictates how geometry works over large scales and gives us insight into the extremely early history of the Universe.

Do we have any evidence that there are other dimensions? (Intermediate)

There currently isn’t any real experimental evidence for extra dimensions. Theoretical physicists are mostly just going by the fact that extra dimensions makes the math work out better when trying to explain certain things, like inflation, the Big Bang, and how gravity works. String theory, one of the leading unifying theories of particle physics, relies on a large number of extra dimensions, but currently has not been experimentally verified. We may also see evidence of extra dimensions if we can observe the effects of our Universe running into another universe in higher dimensional space, which would theoretically cause wobbles in spacetime called gravitational waves. These have yet to be observed though.

What even is space, and why does it exist? (Intermediate)
Fundamentally, “space” is something that allows a thing to exist in a specific place. Our conception of space is tied to how we can be in it and move through it, which in turn is reliant on being able to describe where we are in space. We live in a 3 dimensional space, meaning that we need 3 coordinates in order to describe where we are specifically (x, y, and z coordinate axes, or latitude, longitude, and altitude on Earth). If there is a part of existence that does not have space, then we can assume that it is not possible for things to have positions there, so our spatial concept of things being close or far wouldn’t exist.
This is basically impossible for our 3D brains to think about, but computers don’t really experience the world in the same way we do, so the way they handle space is different, and it can provide an interesting alternative to how we can think about space. If you have a computer simulation of a bunch of balls bouncing around in a box, it doesn’t really know what it means for one ball to be above or to the left of another ball. That box of balls does not exist in a spatial universe, it just exists as a list of properties of a set of objects. To a computer, a ball’s xyz position is a property of the ball in the same way that its color or weight is. “xyz position of (1,2,3)” is a value in the memory in the same way “red” is, it’s just that programmers have told the simulation that if two different balls are both at the same position, then they have to bounce off each other.
So really we can think of space as the thing that gives particles position, and the rules of space tells us how positions behave. All of my atoms have positions in the same way that they have charges and masses, it’s just that we experience the rules of position as “space”. Colloquially, people tend to refer to the vast empty parts of the Universe beyond Earth as “space”, which kind of makes sense because if an area is empty, the only real property it has is its space. The Universe is of course punctuated by various things that actually exist, like stars and planets and such, but the vast amounts of space between these things really define what people mean when they talk about “space”. Our current theories of the Universe make the most sense if we say that space is infinite. We know, to pretty high precision, that the Universe does not curve in on itself, and it doesn’t make sense for the Universe to have an edge, so that means that it should just go on forever, although that’s mostly irrelevant to us since we can only access one specific part of it (the observable universe).
Why it exists in the first place is an impossible question to answer though. The best that I can tell you is that if the Universe didn’t exist, then we wouldn’t be here to observe that it didn’t exist, so if it is possible for us to observe the Universe, then it must exist. This doesn’t really answer the question, but it does mean that existence is necessary for us to even question why existence exists, so nonexistence effectively has no meaning. What exists outside of our existence (either in space or time) is a completely unanswerable question, so feel free to fill in the blank in whatever way makes you feel best, be that religion, science, computer simulation, etc.

Dark Matter

Why can't dark matter just be huge clouds of dust and Kuiper Belt Objects, instead of some other kind of matter? (Beginner)

Many astronomers have considered the possibility that dark matter is not something new and exotic but is instead just a bunch of normal stuff that is too dim to see, but it just doesn’t fit the data we have. The idea of MACHOs (MAssive Compact Halo Objects) making up the missing mass in dark matter halos floated around in the 80s and 90s for a while, but if there were indeed that many planets, small stars, or black holes floating around, we would expect them to pass in front of normal stars relatively often, leading to an eclipse or a gravitational lensing event that we would be able to detect with telescopes. We have seen events like this happen, but not at a high enough rate that it would explain all of the missing mass in the Universe, so there must be something else going on. Furthermore, the models that we have of the Big Bang (which are extremely well calibrated to real data nowadays) predict exactly how much “normal matter” should exist in the Universe, and there is still a large amount of missing stuff left over that we can’t account for, so dark matter seems like it must be something exotic.

If we can't see dark matter, how do we know it exists? (Intermediate)
There is a lot of evidence we have that dark matter exists. There are a number of places in the Universe where we can tell that there is more mass present than what we can just see with light, meaning there must be something unseen that has mass accounting for the difference.
The easiest way to see this is in orbital speeds. When something is orbiting around something else (like the Earth around the Sun), the speed that the bodies move at is determined by how heavy the objects are. If the Sun were heavier, the Earth would have to move faster to keep its current orbital shape. In the 1930s, an astronomer named Fritz Zwicky noticed that galaxies orbiting around in galaxy clusters were moving way faster than they should be considering how heavy the huge galaxies at the centers of the galaxy clusters looked like they should be. The theorized that there must be something invisible that was adding mass to the galaxy cluster and making the galaxies orbit around it faster. This idea was further developed by Vera Rubin in the 1980s when she noticed that the stars at the edges of galaxies orbited around the centers of the galaxies faster than they should, so she theorized that this invisible heavy stuff was present all throughout normal galaxies and made stars go faster than they otherwise should. Since then, we have observed many galaxies and many clusters and (almost) none of them make sense unless we assume that dark matter exists.
Another way we can confirm the existence of dark matter is by using gravitational lensing. General relativity says that light gets bent by gravity in a way we call gravitational lensing, and when we look at very heavy things, we can measure how heavy they are by seeing how much they bend light. When we apply this technique to galaxies and clusters, we see that the mass we measure from lensing is the same as the mass we measure from their orbital speeds, so it confirms our previous conclusions. We know that the dark matter has different properties than the rest of the normal “stuff” because, in specific circumstances, we can see that it behaves differently. There is a pair of clusters called the Bullet Cluster (shown below) that have recently collided with each other and  the galaxies passed through each other (clusters are mostly empty so they can do that). When they passed through each other, the gas in one cluster ran into the gas in the other cluster, causing it to stop in the middle (seen in x-ray shown artificially in red). However, dark matter doesn’t collide with things and we can see with gravitational lensing that it just passed straight through (seen artificially in blue). We know that dark matter must be made of something unique because nothing else we know of would do that.
A third (unrelated) way we know that dark matter exists is by observing the effect of its gravity on the expansion of the Universe. Astronomers have a very good theory for explaining how the expansion rate of the Universe changes over time, but the only way it makes sense with the data we observe is if we assume that there is a bunch of dark matter in the Universe that is different from normal matter. In fact, using this method, we can know (pretty precisely) that around 25% of the total amount of “stuff” in the Universe is dark matter, compared to only about 4% of the Universe that is “normal” matter (the rest is dark energy, but that’s a different conversation).
Despite all of this, though, we still have no idea what dark matter actually is. Physicists have run many many experiments looking for any new particles that could make up 25% of the Universe but so far have found nothing. Maybe one day, though.
What is dark matter made of? I’ve heard of WIMPs and MACHOs but what other options are there? (Intermediate)

Dark matter is indeed very important to galactic and cosmological astronomers since its mass dominates the mass of normal matter and is responsible for forming most large structures in the Universe. The search for what dark matter actually is, though, is more in the realm of theoretical and particle physics. Astronomers observe galaxies, galaxy clusters, and the early Universe to constrain the properties of dark matter and inform future theories though. Theoretical physicists have created theories for multiple different kinds of particles that would explain observations, and now the correct one must be determined. 

MACHOs (MAssive Compact Halo Objects) were an early contender for what dark matter could be (along with WIMPs) and were particularly non controversial because they were just said to be normal things things that were too dark to see (like dead stars, black holes, rogue planets, etc.). They have been ruled out over the past few decades though because 1) even if they’re totally dark, we should be able to see when they pass in front of things, which we don’t, and 2) dark matter has been observed to be essentially non collisional (i.e. it just passes straight through itself) which things like black holes or planets wouldn’t do. So while it is a sensible suggestion to say that dark matter is just normal matter that is dark, it doesn’t fit the picture, so more exotic answers are needed. 

There are a few candidates that are currently popular in the search for dark matter. One of the oldest and most well known are WIMPs (Weakly Interacting Massive Particles). Breaking down that definition, WIMPs are particles that have mass (and thus gravity), but they only interact with other normal particles with the weak force, which is very hard to detect and almost never comes into play. Because of this, they are very hard to detect and many experiments in the past few decades have failed to see any indication of them. 

Another popular particle right now is called the axion. Unlike the WIMP, it is theorized to interact with the electromagnetic force, meaning it can turn into photons in the right conditions. It is theorized to have a mass about 1 million to 100 million times less than the electron, but if there were enough of them, they could still account for the missing mass we observe. An experiment called ADMX (Axion Dark Matter Experiment) is currently scanning for different masses of axions and should hopefully produce interesting results in the next few years.

Another option are sterile neutrinos. You may know about regular neutrinos, which are small inconsequential byproducts of some nuclear reactions. Neutrinos hardly ever interact with anything since they only use the weak force, but they do have some tiny unknown amount of mass; however, the properties of neutrinos are well known and do not match the properties of dark matter. Sterile neutrinos, however, are theoretical “mirror” neutrinos that wouldn’t even interact with the weak force and thus are even harder to detect, but they may have enough mass to make up dark matter if they exist. Theory predicts that they should interact with neutrinos also, but current neutrino detectors haven’t seen any evidence of them. 

So basically, dark matter is still a complete mystery, and whoever can figure out what it’s actually made of will surely win the Nobel prize and revolutionize several different fields of physics and astrophysics.

General Relativity

All the illustrations I've seen of curved space, represent the curvature at the bottom of, say, the Earth. Is it, in fact, such curvature all around the Earth? (Intermediate)

General relativity and the curving of spacetime are always hard to wrap your head around, so physicists and astronomers like to use metaphors when teaching the concepts. The analogy of the bending of space behaving like the stretching of a sheet of fabric that the Earth is sitting on works well for understanding the system in 2D, but unfortunately the Universe is 3D so things have to get more complicated.

Instead of spacetime being a flat sheet, imagine it being a block of foam, and instead of gravity pulling things outward, imagine it pulling everything inwards (like it does in real life). Putting the Earth inside this block of foam will draw a bunch of foam inwards to the Earth, stretching out all of the foam around it and distorting spacetime. This is the effect that the Earth has on everything around it, so it’s not a “top” and “bottom” thing, it’s the same all around. 

Here is an illustration to help you visualize it:


Since the Sun is moving in the sky and its light takes 8 minutes to reach us, does its gravitational pull get misaligned with the apparent position of the Sun? (Intermediate)

When we are talking about movements within the solar system, we must think heliocentrically (centered on the sun) rather than geocentrically (centered on the Earth). The information we receive from the sun is based off of its position ~8 minutes ago, but that is its position relative to the stationary frame of reference of the solar system, not the rotating frame of reference of the surface of the Earth. Even though the sun appears to move through our sky, it is still staying in substantially the same place, so its perceived position doesn’t actually change much over the 8 minutes that the information takes to reach us.

The speed of gravity is another interesting concept that is still not 100% resolved in physics, but we have observed it to travel at almost exactly the speed of light. Special relativity says that no information in the Universe can travel faster than light or else it would violate causality, and this includes gravity. A major test of this occurred in 2017 when UC Santa Cruz astronomers observed two neutron stars merging in a distant galaxy. Despite the gigantic distances involved (140 million light years), the light and gravitational effects from this arrived at the same time, indicating that the gravity was travelling speed of light. The fundamental physics surrounding the propagation of gravity are still not totally solved so it is possible that gravity may travel imperceptibly slower than light, but that is not currently known.

So if the sun were to move or change in the solar system, we would not know until both its light and its gravity reached us ~8 minutes later, and these changes would match up with each other.