Multiverse theory? What multiverse theory? There’s a multiverse interpretation of the equations of quantum mechanics (empirically unprovable). There’s hypotheses out there about other universes with other laws of physics. Nothing observable or provable, though; our universe may be the only one — although it may well have infinite size. Even if it’s not infinite, by human standards it’s basically infinite. There are quintillions of stars in the observable universe. Technically, infinity is infinitely larger, but does it really matter?