This does, of course, bring up the question "What is nothing?" Or what is something, for that matter?
The problem is, anything which existed outside of this universe would have no meaning in this one, as there could be no interaction between the two. Given that, it's fairly easy to argue that whatever is outside of the universe literally does not matter as far as this universe is concerned.
Consider: as stated before, whatever is outside this universe has no meaning in this one. So there's no way to detect anything outside of this universe without leaving it (at which point, even the observer ceases to have meaning to our universe). At our current level of technology, we cannot leave this universe (at least not as far as we know) in order to observe. Therefore, you cannot logically assume that there is anything outside the universe, but neither can you logically assume there is nothing. It is not possible for neither of these to be true, by definition. The only remaining possibility, relative to this universe, is that both are true. It's Schrodinger's Cat on a cosmic scale.