Catastrophe
"Science begets knowledge, opinion ignorance.
There is no reason not to assume something else was outside the universe from the beginning, just as easily as to assume there was not. Supposition requires objective evidence, however, to advance it to a scientific level, of course. That seems impossible for now, though the multiverse ideas are to be relatively respected.
Of course, my flatlander analogy posts do not prove anything, but they give me, at least, a handle on the question "Into whomever else's (observed) universes might we be expanding?".
Cat