“The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. The size of the entire Universe is unknown.” –
By the definition above it is quite obvious that nothing can surround the universe.
If there would be anything that thing would be automatically part of the Universe — because the Universe by definition contains everything. So the obvious answer is nothing(ness). Nothingness surrounds the Universe.
Now about that nothingness…
How can it be different from space? How can the extra-universal nothing be different from the intergalactic, the interstellar space, the interplanetary space, or the subatomic spaces (like between the nucleus and electrons)? The answer is there is no space-time — at least not in any meaningful manner. Space-time has only meaning if you have something to relate to. You can say “one yard from my gate”, or “two years before I was born”, and everyone will know what you are talking about, but you can’t expect anyone to understand if you say “five lightyears from the outer side of the universe”, or “a minute after the Universe ends” because those coordinates are invalid. You can never be “five lightyears from the outer side of the Universe”, because you are something, and the Universe by definition contains everything (including you). For the same reason nothing can happen before the Universe was born or after it is destroyed because if something exists, the Universe exists.
For the same reason there could not be another universe in the same space-time…
Since the Universe contains everything and is surrounded by nothing, there can’t be multiple universes for two reasons:
- They were next to each other.
Since nothingness does not contain space two universes placed into the same nothingness will touch each other. Distance is something, and “every something” is part of the Universe (and not part of the nothingness), so the two universes could not be separated by distance.
- They were the same universe.
Since universe is a set that contains everything, if there are two of them, they will mutually contain each other, and they are exactly the same, therefore they are the same Universe.