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.