The start of the Universe was the start of time.
My personal theory is that when matter enters a black hole, some of that matter (in some form) gets snapped out of space-time, emerging at time 0, at 0,0,0 coordinate. With all the black holes over time, all venting into the same space and time, producing lots of matter in a single space at the same start of time, which would create a big bang.
Just a theory, but one I've not fully proved, nor disproved.
Of course this would mean that the future created the past, and a bit of a paradox, but not entirely impossible.
Slightly different but related thought: what if inside of each black hole singularity is another universe? The stuff it collects ends up in this other universe. Which would mean our universe is just "inside" the black hole of another.
Then perhaps the accelerating expansion of spacetime we detect here has something to do with the accelerating accumulation of mass of our "parent" black hole.
That would add an interesting spin upon multiverses. Without a doubt, black holes and understanding them are very much key to many unanswered and unknown questions.
to me it's more rational to think that each black hole is just punching a hole in our 4-d spacetime fabric just like a sharp point can punch through a bag. what the stuff drains into - I have no idea. Perhaps a higher-dimensional space that contains our universe?
Just a theory, but one I've not fully proved, nor disproved.
Of course this would mean that the future created the past, and a bit of a paradox, but not entirely impossible.