I don't know of any plausible naturalist explanation besides Many-Worlds. And that supposes for the sake of discussion that Many-Worlds is in fact naturalist.
The void in its infinite time and endless space (the same as neither existing) became bored with itself, and in its attempt to destroy itself, split and created the universe we have now. Full of endless wonders and anomalies and beauty and travesty. All for the amusement of itself as one that remembers the abyssal void.
Sadly (?) the word "nothing" seems to have become overloaded, so now—depending on who you talk to—you can have the word pointing to different concepts. See "seven types/levels of nothing":
* https://rlkuhn.com/wp-content/uploads/Closer-to-Truth-Essays...
* https://closertotruth.com/news/levels-of-nothing-by-robert-l...
"Infinity" is another one of those things that used to be murky, but simple; after Cantor we now have different infinities ℵ0, ℵ1... an un-Ockhamian proliferation of terms, and we have to worry about the spaces between them. Science ruins everything!
* https://www.merriam-webster.com/grammar/misuse-of-literally
Could it happen while this universe is here?
What if that supermass is another(the next?) big bang forming; energy just slides around some universe space banging off here and there, forever?
But it does feel like you might have a point here. If everything is moving away from each other, things must have a center some where, and thats where this new big bang is forming?
This is not correct: every object is getting further from every other object it's not gravitationally bound to, at a rate approximately proportional to the distance between them.
I think the unsatisfactory feeling I get from these answers is that nobody ever tries to model worlds with different physics or different physical parameters and try to make them work.
It's one thing to know that everything will break down if parameters of this universe change slightly, but I don't think anyone ever actually seriously tried to make alternatives work, and simply assumed that the only model we know that works is the only possibility.
Of course, I understand it's hard, and we might not have the compute to properly run the simulations to see how things actually work out (without quantum computers, apparently the problem is exponentially hard on classical computers). But philosophically it feels lazy and unimaginative.
Alternative models are being explored all the time. There is incentive to do so, because coming up with better explanations is likely to win Nobel prizes. What is now called the standard model, however, so far explains the existing observations the best, despite being more complex and having a higher amount of arbitrariness than most physicists would like.
I'm talking about consistent models that don't match our physical reality, but that can potentially simulated and which can give rise to intelligent life.
Perhaps 'because' if the consistency did not exist then the universe would fail to exist.
There was the Big Bang, but we do not know what caused the Big Bang. But the particular Big Bang that started our particular universe may not have been the only one to occur. There could have been multiple previous Big Bangs where the 'properties' of each of those created universes may not have had the same consistency as we experience, and the inconsistency(s) could have resulted in a 'collapse' or 'destruction' of those universes.
Whereas it was just a coincidence that our Big Bang got things 'right' for the universe to continue to develop.
We could simply be experiencing survivorship bias in/with our universe.
As someone who dabbles in philosophy, and to use its language, our existence is contingent (we, and our universe, do not have to exist):
* https://en.wikipedia.org/wiki/Contingency_(philosophy)