it's called the theory of the multiverse.
basically, the theory states that an unlimited number of universes exist in the same space at the same time. the reason they don't collide and create an apocolypse that would never end is because they vibrate at different speeds on the quantum level, and then it goes into a complex explanation using theoretical quantum physics and string theory, which is actually quite interesting.
my guess on how the crystal works is it allows people to travel between universes by latching onto bosonic strings and using them to change your quantum vibration. however, if you don't have a crystal and stay too long in the land between, then your atoms are distributed among the many universes until there is less than atomic dust remaining.