Consider a loaf of bread as a 'whole' representing our reality or higher dimensional universe. As we slice pieces of bread from that loaf they become separate pieces of that 'whole' representing parallel universes; independent, yet still part of the original loaf, thus creating a multiverse.
Scientists now believe there may be infinite numbers of parallel universes, and we just happen to live in one of them. These parallel universes may exist less than one millimeter away from us. Our gravity is thought to be a weak signal leaking out of another universe into ours. Travel between different universes via wormholes has also been hypothesized.
These other universes contain space, time and strange forms of exotic matter. Some of them may even contain you, in a slightly different form. Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct.
Physicist, David Deutsch explains the dynamics behind existence in other parallel universes stating, 'To the extent your decisions depend on random events, there are indeed other, equally real, versions of you in other universes, who choose differently and are now enduring the consequences.'
This leaves us with the probability of infinite experiences and realities all occurring simultaneously. The origins of such mysteries as déjà vu, dreams, and other unexplained phenomena could actually be glimpses into our parallel universes.