There's basically an entire sub-genre of superhero comics dedicated to exploring the concepts of alternate worlds. DC's Silver Age was full of Supermen and Flash from different universes either coming to blows or teaming up to fight some inter-dimensional adversary. Jonathan Hickman's current run on Avengers is all about the multiverse collapsing in on itself.
It's not a concept that's unique to comic books, however. The idea of alternate worlds and timelines has been floating about in science fiction for decades, and that was extrapolated from the actual, real-life many worlds theory of time and space, which suggests that there an infinite amount of different universes, almost infinitesimally different from our own. Which means there's a world where Tom Selleck got cast as Indiana Jones, an Earth where Leonardo di Caprio won an Oscar, and a seemingly impossible utopian timeline where Joel Schumacher never got his fingers on the Batman movie franchise, torpedoing it in an explosion of puns and protruding rubber nipples.
Yes, somewhere out there is a world where Tim Burton actually got a chance to complete his Dark Knight trilogy. It very nearly was our world: a script was written, casting was in place, Michael Keaton was still on board, and there was even a title. What else? Well, here are the ten things you need to know about Tim Burton's (unmade) Batman 3.