My impression of the current theories about what came before the Big Bang is that they are simply that, theories. We may never know the answer, as we've reached the horizon of how far into the past we can 'see' with current technology.
However, some scientists posit that asking what happened before the Big Bang is like asking where North of the North pole is - the question makes no sense.
In this rationale, before the Big Bang, space and time didn't exist, so concepts of something coming out of nothing or 'before the Bang' are nonsensical. As well, the math doesn't seem to support a 'before'. It's not that there was nothing, there wasn't even that.
Was it a quantum fluctuation that initiated the Big Bang (which we're still experiencing, by the way), or perhaps our Universe came out of another Universe's particular black hole? I'm not sure we'll ever know.
I'll admit, I could never hope to understand this math, and other peoples' descriptions of what the high-level math actually describes fall well short of the mark. In either case, even if we discovered the actual cause and conditions of the Big Bang, the next question is, in what/where/how did those conditions arise?
It becomes infinitely regressive.