Inspired by io9’s in depth analysis of Hugh Everett’s 1956 Multiple Universes Theory I found this refutation below from Rational Skepticism enlightening. Basically, it seems nobody knows for sure, and there is no way to test the theories. Sounds a lot like religion to me!
The Many-Worlds Interpretation (MWI) is an approach to quantum mechanics according to which, in addition to the world we are aware of directly, there are many other similar worlds which exist in parallel at the same space and time. The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics.
… every time a quantum experiment with different outcomes with non-zero probability is performed, all outcomes are obtained, each in a different world, even if we are aware only of the world with the outcome we have seen.
… Firstly, assuming that there are many worlds like this that exist at the same space & time, it should be acknowledged that these worlds cannot be comprised of the same matter, for if one (the same) set of matter exists at one (the same) set of space & time, then we only have one world, not many. That is, the MWI requires that different matter/bodies exist at the same space & time. But, how can more than one body exist at the same space & time? That is illogical.