Well the short answer is that no one knows, the big bang is the earliest event that we have any knowledge of. But I'll flip the question around, why do you think that just because we are not scientifically advanced enough to completely explain everything about our universe God is even a slightly plausible explanation for the gaps in our knowledge?
Since at least the invention of agriculture a god or multiple gods' power was the explanation for everything: gravity, rain, why the sun rises, etc. As science and technology became more advanced humans found that there natural forces that do not require a God cause these natural phenomenons. Your argument is no more valid than a priest in the middle ages claiming that God must exist because no one could explain how mountains form, therefor God was the only explanation.
Honestly I don't understand how anyone can believe that there is a human like God who created the universe and loves us like his children. I'm not saying that its impossible that God exists, but its also not impossible that there is a giant undetectable teapot orbiting Earth. Both seem about equally likely to me.