Given an infinite amount of time, the universe will expand to infinite proportions. Provided, of course, that "dark matter" is a crock and we aren't looking toward a "big crunch" anytime soon. If our universe is destined to fizzle, though, then it will continue expanding outward indefinitely, possibly colliding with other universes, possibly not. Then there's the argument that there really are multiple universes, but they are not within our "dimension" in a way; that is, they exist, but completely separately of our version of reality, if that makes sense. To put it another way, they are on a separate "plane." We could build a spaceship that travels impossibly fast, beyond the borders of our universe, and explore the void beyond and find nothing at all. The other universe would not be contained within this reality, but in a separate and unique reality, virtually impossible to reach through conventional means.

This is all just pure speculation, but kinda fun to think about anyway.