So, 10 or 20 billion years ago, mass, energy, time, and everything we know about emerged in the Big Bang. What was before or outside of that? The question is meaningless because time was part of what was created, and so was space - there is no "before" or "outside" as far as we are concerned.
Stuff expanded, and after a while, through processes that are not too mysterious, our sun and planet came onto the scene. At some point(s), matter on our planet, which of course was busy colliding and interacting and doing all of the things matter does when you shine the sun at it, happened to fall into a shape that was self-replicating. Naturally, once you start self-replicating, it's hard to stop, and stuff that is better at self-replicating will manage to incorporate more matter into itself than stuff that is less good at it.
Over time, these self-replicating bits got better and better at it through the addition of defensive barriers, the incorporation of other, smaller self-replicating bits, and so on, and after a very long while indeed, many of them were conglomerating together to build absolutely enormous machines to carry them around and help them replicate. (No, no, I don't mean hippie vans. That came later still.)
Some of those replicating machines are us, humans. And because I am one of the humans I can testify that, somewhere along the line, some of the matter started to have subjective experiences. Now, if you think about it, that is just fucking weird. It's hard to even think how to describe subjective experience. If some cosmic overlord machine came along and demanded to know what the hell you were talking about, you'd have trouble being convincing. You start with, "You know how you, like, feel stuff inside? Like you can really tell you're there and stuff? Yeah, me too." If the machine didn't have that experience it wouldn't get it.
So, here I am, a gene vehicle, on this little piece of space dust. In 100 years, I'll be at best an old photo in someone's family photo album. In 1000 years, nobody will remember a damn thing about me. In 100,000,000 years it's extremely unlikely anyone will remember my species. And sometime after that, there won't be any life on this planet, and sometime after that, there won't be any planet earth, and not only will nobody remember us (not even our best art and most fantastic thoughts and culture), nobody will even know that we were forgotten. And eventually, one way or another, the universe will devolve or crunch up to the point that there will not even be anyone of any kind left to know or not know anything whatsoever.
There could be other parallel universes, whatever that means.
These questions and the answers given by science don't really make sense to me. But then, why would they? My brain is evolved for life on this planet. There's no reason I would have the mechanisms needed to understand the nature of reality itself. Nevertheless, I do find the whole thing implausible.
One could embrace a religious view instead. The ones I'm most familiar with replace the grand mysteries of the material universe with a single mystery - some god or gods. We have no evidence for these deities, but a big controller entity is in some sense more workable for my socially evolved brain than a bunch of causeless, purposeless, endless, meaningless universe. I can grapple with an entity. I am one myself. I get that.
It'd be really nice to wake up from this weird-ass life at some point and get some answers. I guess if that's what happens at death, I'll find that out at some point. If, as I suspect, nothing happens at death, then I just won't ever know it (or anything else, ever again).