“Go, then. There are other worlds than these.” -Stephen King
We think of the Universe as all there ever is, was or will be. But, in fact, there’s a limit to the most distant galaxies, stars, matter and radiation we can see. The hot Big Bang occurred a finite amount of time ago, and hence the amount of the Universe accessible to us through any observational means is necessarily limited.
What lies beyond that? According to our best explanations and theories, there’s more Universe. But beyond that, the combination of cosmic inflation and what we know about quantum field theory indicates that there are multitudes of individual pockets that contain entirely disconnected Universes, each beginning with their own Big Bang, some of which may even have different fundamental laws and constants from our own.