And WHAT THE HECK is the universe 'expanding into'?
The answers to these questions make me dubious on tiny-human understanding of the nature of reality.
Mechanical Elves, anyone?
Imagine an RTS game on an infinitely large map with fog of war. Sure, there's something out there beyond the fog of war, but we can't see it. (don't try to go too far with this analogy though)
The universe isn't expanding into anything. It just is. This is the key to your misunderstanding I think; the universe isn't expanding through or into some other, extra-dimensional medium. The universe itself is expanding.
My understanding was that the universe originated as a point, inflated a great deal, then began expanding at a rate that has changed over time. As a result, the universe would be significantly larger than the visible universe, but would have a finite size.
(A finite size, but no boundaries. The universe is a sphere whose circumference is everywhere but whose center is nowhere. Cue Borges music.)
Rather everything was just closer together. Imagine traveling back in time - you would see galaxies move closer and closer - until everything was super dense and hot - like the center of the sun but everywhere.
It would. It is not an unanimous conclusion that "stuff" is found everywhere in the universe and that the universe is infinitely big. However, that doesn't necessarily imply a center or an edge.
The universe might be infinite and unbounded.
The universe might be finite and bounded.
The universe might be finite and unbounded.
If the universe is finite and unbounded, we can go back to the ants on a balloon analogy. As expansion continues (and indeed speeds up) the balloon will continue to inflate, for forever. It is not possible to ever make the trip around the balloon to get back to your starting destination due to the expansion of the universe. The universe and spacetime are the 'surface' of the balloon - there is nothing inside of it, the center is not the middle of the balloon, and you cannot tunnel from one side of the balloon through to another. That isn't to say the observable universe is likely to be the extent or even close to the extent of the actual universe - it is likely still just the tiniest of fractions of the actual universe.
We're probably finite and unbounded, and expanding into nothing. As the universe expands, it creates space where it did not exist.
There's other theories that are reasonable, however. Some posit that our universe is a bubble in a true vacuum, and that there could be other bubbles containing other universes. Those theories are largely driven by the theory of inflation, which is the most accepted explanation as to how the universe expanded so far and so quickly in its early stages. (That's not to imply that the bubble multiverse theory is the most accepted explanation, though - just that most theories relating to bubble multiverses stem from looking at vacuum energy)
We probably won't know for certain in our lifetimes. Intelligent lifeforms might never know for certain.
Wouldn't it be more accurate to say that everything was at the exact center of the Big Bang? Assuming it started from a singularity (which can't really be known), everything was in the center.
I also really wonder what is the universe “inside of”? Like why is spacetime here in the first place?
I agree that our explanation must be impoverished compared to the true nature of nature. It really boggles my mind. I enjoy the public physics lectures scattered around YouTube, but it all still leaves me realizing how limited our understanding is compared to the mundane goings ons of our universe. Our current understanding is that we’ll never communicate with distant galaxies, meaning there’s likely a trillion inhabited worlds full of beings we will never know.
When we talk about the universe we're rarely talking about what do not know yet, but rather what is possible to know.
Whether there is another universe "outside" of ours is not just something we don't know. It's something we cannot know but it's also something that cannot matter. The observable universe is, by definition, the total encompassment of everything that can possibly ever matter to us in even the strictest mathematical sense.
And by the time our radio signal gets to them, the ones there now will all be long dead, interpreted by some future generation. By the time we get the response, humanity may be long extinct. So for all intents and purposes, we may as well be alone in the universe.