While the QM theories are able to present the math that would imply the existence of a multiverse, there is no way to PROVE the existence. Since a proof can never be found, it falls short of scientific theory. I like to think that the multiverse exists, it pleases my brain and teases my sense of awe and wonder.

M-Theory (see Ed Witten at the Institute for Advanced Studies - www.ias.edu/scholars/witten ) supports the Multiverse theory. It makes sense (what little I do understand of it) to me. I personally think this guy is probably the smartest guy on the planet. After all he 'unified' several 'conflicting' string theories at a presentation at USC yrs ago.

There is one infinite universe

As far as I can tell it's more of a hypothesis. But a good one. It hasn't been disproven, and from what I've heard has been a helpful framework for considering modern cosmology, so that's plus. But so far I've not heard of it really putting out much that has been (or possibly even could be) validated. Without anything which looks predictive or validatable, I have a hard time elevating it to the label of theory. It's nowhere near Einstein's theory of general relativity, say, in that regard.

That being said, its' mathematical formulation and ability to condense into other cosmological models put it in a better position in my mind than "God did it." Multi-verse theory is potentially a little better than that, though not much in its' current evidentiary state.