Last I heard, the "multiple universes" thing in physics was a way of doing the maths to make the quantum mechanics drop out with a sensible answer under certain conditions, because of it all using probabilities instead of known states. This doesn't necessarily mean the universe does it that way.

As a ferinstance, suppose I go to my online bank and transfer £10 from one account to another. This means that I can now take out £10 more from the second account. But it doesn't mean that a physical £10 note has ever moved location.

Graham.