Let's get this out of the way up front: I'm no physicist and have no business proposing any of the things I'm about to propose. The likely outcome here is that someone with relevant education can easily expose my musings for the silliness they are and maybe teach me something in the process. That said, I have a hypothesis that very tidily explains dark matter, primordial black holes, and where all the antimatter is. All while answering the question, "what existed before the universe?" I'll give you a moment to stop laughing...
First, antimatter. If the fabric of spacetime really does behave something like an actual fabric, what's on the other side of it? If you were somehow able to breach it, what would you see? What if it were a parallel universe made entirely of antimatter? What if that's where it all lives, just on the other side of spacetime itself?
Now imagine that we're far into the future of our universe, in a period where supermassive black holes have consumed most of the matter in the universe. Over countless eons, some of them might eventually cross paths and collide, forming some of the most massive bodies that could ever exist. Could they become massive enough to breach spacetime itself and, for a brief moment, come into contact with their antimatter counterpart on the other side? Wouldn't the result be an explosion and dispersal of matter to rival the Big Bang itself? What if the Big Bang was, in fact, just the most recent occurrence of this? What if our Universe cycles through a never-ending series of coalescing and dispersal in this manner?
But how much matter would it take to breach spacetime, assuming that what I've said so far holds true (I'm sure it doesn't, but bear with me)? Would it require all the matter in the universe, or just a large enough chunk of it? Let's assume the latter and suppose that the supermassive body responsible for our Big Bang represents, say, 75% of the total mass of the universe. Where did the rest go? If it too had coalesced into black holes, the Big Bang would have scattered them like billiard balls. Their mass wouldn't have gone anywhere, it would still be just outside the boundary of our known universe in the form of supermassive black holes larger than we'd expect to find. This may be all that dark matter is: supermassive black holes left over from the previous incarnation of our universe.
Now let's assume that primordial black holes can be proven to exist (to the best of my limited knowledge, they're still purely hypothetical). If the universe really is much, much older than we think, these could simply be stellar black holes that have mostly evaporated.
Thank you for bearing with me through what has likely been a mess of uneducated, ridiculous rambling. You may proceed to tell me how much of this I've gotten wrong, and why.