As I said, there are varied hypotheses about possible causes, Branes are one as suggested by Steinhardt and Turok. But evidence is lacking at the moment so work is ongoing. The truth is that we may never know.
The Universe, which includes space and time originated at the Big Bang. This is supported by the evidence from the Red Shift and the CMB (among other things). The scientific evidence is there to show that the Universe started to exist in its current for which means that there was non-existence. This however was not what yopu asked for, you asked for evidence of the cause.
The point about "prior to" is not silly, it is vital to understand some of the concepts that you realise that as time itself is part of our universe a lot of human concepts relating to that area become meaningless once we approach the very start of or universe.