In the Multiverse/Multidimensional universe theory, all of these dimensions aren't necessarily in different universes. Some are of different dimensions, laid over top of ours, but we can't see them because we can't perceive the dimensions they exist in. The only dimension that bleeds into ours, that we can track so far, is gravity. This is what we call "Dark matter". It is actually gravity leaking over from an "Nth" dimension laid over our own that we can't perceive.

Again, as to the veracity I can't say. And I wish I remembered to author, but I don't =/ maybe I'll look it up later and make an annotation.