Our universe may be one of many. Instead of a single reality where everything exists in just one of its possible states, the existence of a multiverse is the idea that everything possible does exist all at once. Try and get your head around that one. What colour pants did you wear today? All of them. 
 
There are a few theories for how multiple universes could happen.
 
If space time is infinite yet the particles within it are finite, then at some point space time must repeat. That would mean there are other versions of ourselves out there doing all the infinite other things we could be doing. Finally, the washing pile somewhere out there is done.
 
Bubble universes are the idea that parts of the universe kept expanding after the big bang. Our universe stopped expanding and stars and planets formed, creating its own little bubble in a sea of universes. But other parts of the universe kept expanding, and who knows what’s happening out there. Not me.
 
Parallel universes come from the possibility that there are more dimensions than just the three we see, along with our experience of time. Sometimes these other layers of universes could even slam into each other and reset the universe. People keep comparing this to sliced bread, which is the weirdest bit of all these theories.
 
But really, aside from all the quantum mechanics, perhaps the most convincing argument that a multiverse exists is that we can’t prove it doesn’t.