The physicist, Stephen Hawking, offered substantial evidence that time as we know it started from the singularity of the Big Bang.
No, he did not.
It is misleading to say "what happened before the Big Bang" because this implies time continuing to extend back.
As I said, we simply don't know because there is no tested theory of quantum gravity and that
will have an impact on what did or did not happen at the time general relativity predicts a singularity.
In any event I was talking about before the time that our tested theories allow us to go back to (when quantum gravity effects would become significant). I'm not denying that that it's perfectly possible that time started at the BB.
It may be more accurate to consider what happens or exists outside the Big bang, or more accurately, outside our universe. Outside the universe could be an existence which is aware of this universe and its time dimension, but not part of it.
And gravity
could be caused by little purple, multi-dimensional pixies, all called Eric, that pull at the fabric of space-time....