My friends, the idea of "multiverse" is just a hypothesis from quantum physics and is entirely unproven to become accepted theory... i.e. it is just a series of mathematical formulas applied to a concept on some of the potential attributes for any universe's "inflation" principle to have functional origin and conclusion. Energy, light, and time.
In some circles of science this has explanations for but also consequences in how we observe the universe we have. One principle with a glaring consequence is the "observable universe" only accounts for how far light has traveled since the origin of the universe, if you apply "big bang" or "inflation" principles to the equations then the immediate hypothetical argument is everything outside the universe is either space ready for or already used by something else.
Quantum sciences and physics suggest through math alone that if our universe has these properties that we account for with laws of physics, even if unequally applied, then we have two potential conditions.
One, there are alternate universes out there that either exist with similar space-time principles that are separated by area that light has not reached from either universe or something very similar in space-time principles where light may have crossed but not observable. today. The idea being a gap of darkness that from our position our universe and its 13.7 billion or so years of age has not "inflated" out far enough to see another universe to explain the former.
Two, there are alternate universes out there with different space-time principles that are separated by the laws of physics confines of each one and may exist right on top of each other or in some way parallel.
Either way the immediate response is confines of our scientific limits today simply because of where we are in human evolution from Earth observing things and applying math. One may respond with quantum fluctuations as explanation for the second option but it matters not to the discussion in hypothetical terms because beyond light we have no observable facts to suggest what our universe is really shaped like in extreme, only what we can observe based on how far light has traveled.
You could argue well, and I would, that we are in borderline science fiction area now because the principles from these mathematical formulas are just that, applied math to explain a hypothesis we are incapable of proving exclusively because of our own limits.
Perhaps down the road discoveries will clarify these things, and that is the good news about science... it tends to provoke additional efforts to expand our knowledge and clarify or correct prior understandings. Until then... damn near science fiction but plausible.
