Correct me if I am wrong, but I believe both String theory and the Theory of relativity allow for parallel universes. I think they exist personally, but I'm not sure of the idea of countless universes (though that would be perplexing and fascinating)
String theory by definition is theory.
While I also subscribe to many aspects proposed in String theory and do understand the theory of relativity nothing is proof in the sense of scientific method.
There is a theory, not sure what it is called, that postulates that time is a series of static Universes and consciousness is the movement from Universe to Universe. That the consciousness is non-dimensional and not restricted to directional linear progression. In essence, no defined singular path but all paths at once. The idea is the basis of simple quantum mechanics. That multiples of destined paths occur simultaneously.
That any given observed path is but one of an infinite number of paths that exist in a matrix of infinite dimensions. Each one with its own consciousness but unaware of all the others. In essence, separated from all the others by dimensional boundaries.