In what sense are you referring to a multiverse? Because the many worlds interpretation is a valid phenomenological approach to wave function collapse, and there’s just as much evidence for that as for the Copenhagen interpretation. Consigning something like that to “science fiction” is a bit premature, don’t you think?

Even if you’re referring to other theories of a multiverse, like Roger Penrose’s conformal cyclic cosmology, mathematics is a very valid approach to the world. It’s like you think mathematics doesn’t tell us things about the natural world. We largely developed our quantum field theories by investigating gauge symmetries, so why is developing a theory based on the math of conformal invariance such a stretch? I get that it’s difficult to verify, but “science fiction”? That’s jumping to conclusions for which there is no evidence.