We can measure the expanse of matter into space-time.AND seeing as we can measure how large our universe is.. I am pretty sure it is not infinite... and since space is not infinite time cannot be either.
The matter of Our know universe is expanding into the infinite amounts of space-time.
Whos to say there wasn't another big bang trillions of trillions of trillions of etc light years aways from Our known universe?
Just as there are + billions of galaxies in Our "known universe", there could be billions of "unknown universes" in the infinite space-time. When I say "know universe" I am referring to the edges of know matter in Our + billion galaxy big bang.
So there could be big bangs taking place all the time in parts of space that are in unfathomable (and unreachable) parts of infinite space.
Our known universe compared to the unknown universe could be like a cell compared the rest of the known universe.