It's complicated...

Some compare it to the surface of a balloon. Mark 2 points on the surface. When you blow it up, they will become further apart. A 2-dimensional creature on that surface would see their universe expanding.

That 2D creature would not be able to visualize the surrounding 3D world anymore than we are able to imagine more than 3 dimensions. It might dream of taking a short-cut to the other side of the balloon by building a bridge through the center. That's the Sci-Fi fantasy of hyperspace travel.

Mind that scientists don't believe that the universe is like a 3D balloon. A balloon has a closed surface. You can go round and get back to where you started. The universe is thought to be more like a flat rubber sheet being stretched where no one can see the edges.

You might now wonder what kind of high-dimensional space the universe exists in. But even that may be a completely wrong way to think about it. This universe is all we know and have ever experienced. Worse. We evolved in it. We are absolutely constrained.

Let's take a little detour through Einstein's relativity. Imagine a spaceship travelling at close to the speed of light. It flashes its headlights. The light moves on ahead of the ship at, of course, the speed of light.

So here's the thing to break your mind:

You look at this from the outside. You see the ship moving at close to the speed of light and the flash slowly gaining distance. The ship is almost as fast as the light, right?

You look at this from the inside. You see the flash of light moving ahead at... the speed of light. You don't see it moving just a little faster than your own spaceship.

Everyone, everywhere, always sees light moving at the speed of light. The person in the spaceship sees the flash moving away at the speed of light. The person outside, at relative rest, sees the distance between the flash and the ship increasing only slowly.

How can this work? Well, time needs to pass slower on the spaceship relative to somewhere at relative rest. That's what time is relative means. There are more things that need to give way, like space/distances.

Intuitively, we think of time and space as absolutes but it is not so. Light is an electromagnetic wave. So we might think that it behaves like a soundwave or an ocean wave. Not so. The speed of light is fundamental and time and space are built on it.

Actually, the speed of light is the speed at which electromagnetic phenomena spread. We believe it is the general top speed of things happening in the universe. When you move a magnet, then the magnetic field around it moves. But this movement spreads only at the speed of light. The gravitational field around it also moves, but also only with a delay given by the speed of light.

Let's go back closer to home. Everything consists of atoms. Atoms have a positively charged nucleus and a negatively charged shell of electrons. What keeps the nucleus and shell together is electromagnetism. The reason you can't walk through walls is that equal charges repel. Electromagnetism is how our reality is solid.

We intuitively think of everything as space with stuff in it. But that is a playing field created by electromagnetism and a bunch of other things.

This view obviously starts breaking down when we go away from what we are used to. At large scales and high speeds, you have to think in terms of relativity. At very small scales, it gets quantum. Stuff only has a location, speed, or other properties in interaction with other stuff.

Quantum theory and relativity contradict each other, but also are completely accurate as far as anyone can measure. The conditions where they'd contradict each other exist around a black hole or maybe at the beginning of the universe.

Let's get back to the question...

You ever use a spray can for a while and notice that it gets cold? That's how a fridge works. And also our universe.

When everything was closer together, the universe was hotter. About 13.8 billion years ago, it was white-hot glowing plasma. Plasma means that the atoms bump into each other so hard that they knock off the electron shells of each other. When there was enough room/the universe had cooled enough, this stopped.

The light of that is what we now see as the cosmic microwave background radiation (CMBR). When that light started out, all those billions of years ago, it was quite close to us. But the universe expanded and so it had to travel a long way, eventually. The expansion of the universe also caused the wavelength to become larger. The wavelength expanded like any other length. So instead of light (nanometer wavelength) we now have microwave radiation (micrometer wavelength).

When we look out from earth, we don't just see into the distance. We see into the past. That glowing plasma is opaque to light, so that's as far back as we can see. It's the edge of the observable universe, but it's not a barrier to which you could travel. It's in the past.

We assume, and it appears to be true, that the laws of nature and thus the speed of light, are the same everywhere. That means that the edge of the observable universe is a sphere around us.

As far as we can tell, every point in space expands equally. The CMBR comes uniformly from everywhere. We don't know what this expansion means or why it happens. We only know that we can explain the observable universe with the Big Bang Theory. Maybe it's just some fudge we are stuck with, because we can only think in terms of time and space.

• Time is a function of our universe. Before our universe, there was no time.

• If there is no time, there is infinite time.

• If there is infinite time, everything will happen.

• If everything will happen, our universe will exist.

Thus, here we are. Or, to paraphrase it in more famous words - if the universe was not as it is, we would not be here to see it.

We don't know 🤷
One of many remaining mysteries of the universe is what expansion really means. We can measure that it is indeed expanding, but we don't know what's driving the expansion (other than "dark energy" which is actually only a placeholder in equations). We also have no idea about the size of the universe. Observable universe is just that - as far as we can see. But there is more beyond that we can't see. There is no observable border to it, but we also can't claim it's infinite because there is a lot of proof for expansion.

Space is everywhere, and it's stretching. It isn't expanding per say, it's that everything that is everywhere is getting further apart.

You're thinking of space stretching like an inflated balloon, and that's a decent analogy but it's far more simplistic than reality. If there is some cosmic boundary it would divide everything that is and exists, and the absence of any existence. It's a boundary which light itself could not escape, and all future points of time exist only within that boundary. All of space time would fold back in on itself.

Imagine standing at that boundary and firing an arrow, a thought experiment first described by Lucretius in Pompeii 2000 years ago. He imagined that if he fired an arrow from the edge, then clearly the universe kept going, and if it hit something, then clearly something must exist on the other side.

But the arrow can only fall back in towards the universe at some such theoretical boundary. Because all points in time exist within that boundary.

Which is very similar to a black hole. Which is why some cosmologists have suggested that maybe we live inside one.

I've seen it descrived as getting inflated, as in, every single point of it getting further apart.

This makes it so there's no "nothing" to displace, there's just more new stuff.

My theory is that the universe is a higher form of a sphere. If you travel along a straight line long enough, you end up in the same place you started.

There is the analogy of a balloon's surface where every point moves away from its neighbor, or a better analogy of bread expanding as it is baked, since that's more three dimensions. The idea is that space is expanding at the atomic level at a certain rate, but it's so small that it takes an astronomical amount of these atomic increases to be able to measure it (we can't measure expansion at solar system scales, or even between our galaxy's stars, as gravity drowns out the effect. But space is so large that over distances like between galaxies, the light that has traveled all that way has had to travel over this expanding so much that we can see a shift in its wavelength. And overall everything is shifting red, so either we in our section of the galaxy are the center, or it's something that's common in any part of our universe. One of these is far more likely.