system
Given the following post, respond as if you're commenting on Reddit:
human
What's outside the universe?

So firstly im just some dumb 17 y.o with a question, and the question is... So space between all the objects in space is a vacuum of nothingness or spacetime or whatever but what is beyond that, that the universe is expanding into. Is there no space no time laws of physics???
gpt
This is a very interesting question! And part of the answer is that physicists use the word "universe" in multiple ways that can be quite confusing. 

First off there's the "observable universe": the area of the universe that we are able to see, because light or other signals have had time to reach us from there. This is a spherical region centred on us, with a radius of about 47 billion light years. [Important note: the fact that we are at the centre is not special, people in a neighboring galaxy would see a slightly different observable universe, centred on them.]

And then there's what we could call the "full universe" (not a technical term): the whole of everything that exists, even the stuff outside of the observable universe that we can't see. At the moment the evidence we have seems to indicate that this is infinite, it keeps going on forever and ever. Certainly, our observable universe is only a tiny fraction of the "full universe". 

Within our current understanding of physics, there is nothing "outside the (full) universe", in that the concept of "outside" doesn't really apply. If the universe is infinite, then it never ends, it just keeps going. And if it's finite then it loops back around on itself, the same way you'd end up back at your house if you started walking North and kept going in a straight line for long enough. There's no "edge" either way. 

Now, within theoretical physics there are models that consider our universe to be embedded within some kind of higher-dimensional space (the way the text on this 2D screen is embedded in your 3D room), and you could consider that a kind of "outside" but there's no evidence at present to support that idea. 

To answer "what is the universe expanding into?" is tricky. The answer is straightforward: nothing. But explaining why is hard. When you blow up a balloon, it expands out into space that was occupied by the air, we naturally understand this. But that's an example of a higher-dimensional embedding: the balloon's rubber is a (curved) 2D surface embedded in 3D space. In the case of our universe, it isn't pushing out into some external space, it's just that the distances between very far apart galaxies are getting bigger.