That's a good question? If our universe is finite, like the balloon analogy suggests, what is outside our universe? I'd love to hear some physicists ideas on that. Does anyone have Stephen Hawking's email address?
I Googled "what's outside the universe" & read some interesting articles on geek.com and askamathematician.com
Some of the answers were:
1. nothing, our universe is all that there is (even though its finite)
2. Our universe is infinite (in which case there are major flaws with the balloon analogy)
Interesting quote: "If the universe is infinite, it would also contain an infinite amount of matter. In this case, literally every possible arrangement of matter is present an infinite number of times. There are an infinite number of Earths out there, if we look far enough afield, some drastically different from ours, some virtually identical, some literally identical. Actually, there would be an infinite number of every one of the infinite possible Earths. As to what’s outside this universe, well, there’s obviously nothing beyond an infinite border." Mind boggling.
3."then there are the multiverse explanations. These postulate that the universe split off after the Big Bang into everything from bubbles to sheets. Our universe is just one of many, possibly a finite number or possibly infinite. In this conception, what’s “outside” our universe is simply another universe. It could have identical physical laws to our own home, or have completely different ones. Everything from gravity to the strong nuclear force could be different, leading to a reality that could behave differently in fundamental ways"
-But then your question still has relevance; what medium fills the space between these universes?
Cosmologists have the best jobs. They get paid to sit around and think about this crazy stuff. It makes me realize how much my job sucks. Sorry for the long reply.