Our Big Bang "may have been" a super-massive white hole event, "so we are" a universe-within a universe. Hardly an irrefutable hypothesis. You need to

Our Big Bang "may have been" a super-massive white hole event, "so we are" a universe-within a universe. Hardly an irrefutable hypothesis. You need to revise the "so we are" into "so maybe one might imagine we could be" in light of the "may have been" part. But taken as granted, let's just say the big bang actually happens to have been what you describe. The first thing that such a hypothesis would reveal is that in such a scenario, the big bang we consider to be as an event of origin and the beginning of "space/time" suddenly is not at all the beginning of anything. It is no longer the origin of the universe. Now, there was something prior to it. Second, if we look upone the universe, as it exists today, we find many super-massive black holes. At this point it becomes clear, that there are likely to be, or to have been just as many super-massive white holes. All contemporary, and all distributed in a similar fashion. If there's one, there's another somewhere else. Just like we see with galaxies, or exotic stars, or exoplanets. If there's one, there's another somewhere else. So too for white holes. Thus, even if the big bang, which we see as evidence in the cosmic background radiation, remains the most recent grand-daddy and mother of all cosmic events in the past 20 billion years, it cannot be assumed to be unique in THIS unerverse, and in THIS space-time continuum. We can only draw the conclusion that in this very universe, there may have been another super-massive white hole event, prior to even the big bang, just as transformative in the distribution of mass and energy, and not only that, there will be a similar cataclysmic white hole event in the future, perhaps beyond the era of humanity as we know it, whether we continue to reside in this solar system and galaxy or not. So, multi-verses are not a necessary conclusion to draw from white hole events, because there's nothing stopping total creation and total destruction and then total recreation from occurring and reoccurring in the same universe we seem to be in right now, simulation or not.