Well, this question has come up before among a circle of my friends, sitting around being contemplative late on a Saturday night. Of course, none of us knew the actual answer, but that didn't stop us from giving the matter some thought.

The view we settled on in that discussion went like this:

Space itself is infinite, but the matter in it is not. So when we talk of "the universe," are we talking about space, the matter in it, or both? That's the first thing to settle. We decided that it's both, since "the universe" is defined as everything that exists.

So then we decided that when people talk about "the expanding universe," it's really a misnomer, or maybe a convenient shorthand, for the concept of "the expanding of finite matter into an infinite space."

Which makes sense when taken together with the Big Bang theory (if one accepts it): all the matter that exists started in one place, and bang, it started expanding outward into the infinite empty space... and it is still expanding outward.

I have even heard a theory that states, at some point, all the matter will stop expanding and start falling in on itself again, leading to another Big Bang. Which further suggests that we may not be in the middle of the "first" Big Bang, but that it's a zillion-year-long cyclical event that may have already happened many, many times.

I think it's just tough for us little finite creatures to really understand the concept of infinite space. I mean, everything we deal with has edges to it. There is a place where an object stops, and something else starts. So, it's sort of natural for us to wonder, "What is beyond the edge of space?"

But if space is truly infinite, it doesn't stop or start, it has no edges. It is EVERYTHING... and that means it doesn't expand. Which is just totally alien to our everyday frame of reference.

Phew. Thinking is good exercise.
Thanks for the opportunity, Kat.