The multiverse hypothesis is strictly in the realm of theoretical physics at the moment- all calculations and educated guesses. There is some promising data coming out of CERN and in cosmology that suggests the idea could be on the right track, but a LOT more evidence is needed before we can really say anything about … Read more
They've been postulated but nobody really knows. If you visit the link below and scroll down to the heading that says "What about other universes?" you'll find what's known.
On the other hand if you're talking about the "multiverse" which contains other versions of our planets and ourselves, that's been speculated … Read more