Eternal return theory is the idea that the universe has been and will continue to recur.
Assuming time and space are infinite, imagine the probability of another world like ours existing, where everything plays out exactly like it has here on Earth, with me writing this post and you reading it. If "a mathematical certainty" wasn't your first thought then like myself you probably don't have as good of an understanding of infinity as you may have thought.
Another example is monkeys typing on a keyboard until a copy of Shakespear's Hamlet is produced. The likeliness is extremely low but through infinite time it's bound to happen.
I'm sharing this idea just because I like it and want to know what other people think of it.