Longer answer: Maybe nothing. Maybe it's nonsensical to think of something as being outside of the universe - as it expands, it creates space where there was none. Maybe a true vacuum that our universe is floating through. Maybe other universes. Maybe something else.
We might not be able to ever know the answer to this question - the laws of physics might prevent us from being able to decipher it.
Longer answer: Maybe nothing. Maybe it's nonsensical to think of something as being outside of the universe - as it expands, it creates space where there was none. Maybe a true vacuum that our universe is floating through. Maybe other universes. Maybe something else.
We might not be able to ever know the answer to this question - the laws of physics might prevent us from being able to decipher it.