May l suggest a book by Dr. NFJ Matthews. It's not for everyone, you'll need a high level of understanding of physics and the math that goes with it. [amazon.com]

I think all of time is happening at the same time and I think it's possible that events like the big bang are always happening at the same time. I think quantum particles link everything together and that could account for them seeming to appear and disappear. They could be like a needle weaving in and out of realities, tying everything together while conventional physics keeps each reality with respective matter apart. I know one theory uses the analogy of our universe contracting and expanding like a lung. It's possible those actions create friction, helping a big bang happen somewhere else in our reality or maybe in another one. I like thinking about these things. The possibilities are endless.

Our universe, and all of it's features, started with the Big Bang. Of course there could be something that came before it. But, I don't think the same laws of physics need to apply. Perhaps we could some day extrapolate what came before, but why bother? It's not a universe we live in, nor could we. I think a universe of some type has always existed, it's just that the universe as we know it, has a beginning.

Obviously space-time hence time started with the Big Bang so your question asks what was before time existed. So yes the question, at face value, is nonsensical but the mind can go beyond the paradox and understand that before is both a physical place and a concept outside of time.