Spherical Universe

The universe is not expanding out from a centre into space, like an inflated ballon, rather, the whole universe is expanding and it is doing so equally at all places, as far as we can tell. Your question is the result of a misunderstanding, but the misunderstanding isn't your fault; rather, it's the fault of the famous (or more accurately, infamous) "balloon analogy" for the universe's expansion, which, in my humble opinion, should be banished forever into the dustbin of history because it's the source of so much confusion.

The problem with the balloon analogy is that it's a two-dimensional analogy for a three-dimensional situation. The way you're supposed to think about the balloon analogy is that everything which happens in two dimensions on the balloon's surface actually happens in three dimensions in the universe. For example, the balloon's surface "stretches" proportionally in TWO directions as the balloon gets blown up, but our universe stretches proportionally in THREE directions.

The balloon is the planet earth, and everything beyond it is the expansion. I think a good word for it is "infinity."