For a long time, I’ve pretty much taken it as a given that there are or at least have been other Universes. Our universal constants (I think the relative magnitude of the dozen or so Planck units determine everything about how a Universe behaves) are the kind that allow atom formation, accretion into stars, nuclear fusion, and supernovae that form heavier elements, forming solid planets and life. It seems like there would have been other Universes that didn’t meet these criteria, resulting in Big Crunches, heat deaths etc. For example in a Universe where gravity were more reliant on distance (distance cubed instead of distance squared), stable orbit would be possible only if it were perfectly circular, so solar systems would be very unlikely to form.
Anyway, I’ve had to accept that the existence of other Universes is an almost necessarily conjectural hypothesis, since as far as we know, Universes don’t interact. With no way to observe or test their existence, it’s not a very good hypothesis, and I should probably not assume anything other than our Universe exists.
However, if multiple Universes are the best explanation for something like universal background radiation anomalies, they don’t have to be conjecture. What I’d gathered was that the oft-discussed anomalies were just the leftovers slight burps in our Universe’s inflation stage, or something else early in the Big Bang. I’m not big on the specifics, but the anomalies are just the leftovers of a non-uniform Big Bang; a uniform Big Bang would have produced perfectly distributed matter, and without some noise, the Universe would still be a uniform ether of matter and energy, either thinning out forever, or being pulled back into one Big Crunch singularity. As far as I know, laws of physics and things like “time” and “gravity” didn’t fucking exist at the Big Bang (or “before” it, if that bears any meaning in this context), so who knows how other Universes may have influenced the formation of ours? I wish I understood enough theoretical physics to read the research paper.