In book Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos Brian Green shows that many current physical theories (e.g. quantum mechanics - many world interpretation of QM, superstring theory or theory of inflation) allow a possibility that our Universe is a part of bigger structure - a multiverse.
He also added some other theories like a holographical principle or simulated universe (meaning universes simulated on computer) to the list of theories above.
However, the issue is that many of these theories presented in the book predict that it is impossible to verify that the multiverse exist because universes cannot communicate each other.
My question is: Are theories of multiverse really scientific theories if we take into account that their predictions cannot be verified? These theories seems to me to be rather philosophical concept than serious science. Or do I miss something?