Extra dimensions are pretty much the definition of unfalsifiable.
String theory is pure science fiction. It has no relevance whatsoever to the real universe.
String theory and dark matter and extra universes are mysticism. That an conceptual paradigm of the universe should require these things is as clear a sign as one can get that the paradigm is wrong.
Must be nice to have such certainty. Given a universe of many different dimensions where we are stuck living in 3 of them it such does a nice job of explaining what we are seeing but to each his own.