What happened before the Big Bang?
The current Big Bang theory literally requires that something existed before the hot big bang for the physics and math to work out, due to the observed smoothness in temperatures and densities. The Big Bang theory timeline starts at time t0 but the hot big bang doesn’t occur until time t0+10-32 seconds. What occurred at time t0? We have absolutely no reason to think anything of importance occurred at time t0. t0 is simply the farthest time we can extrapolate backwards to. The time between t0 and the hot big bang represents the final fraction of a second of the previous phase of the universe which had an unknown length that could possibly have lasted billions of years and is generally believed to be “cosmic inflation” but could also have been a Big Bounce.
https://bigthink.com/starts-with-a-bang/universe-infinite/We can only see the observable Universe created by inflation’s end and our hot Big Bang. We know that inflation must have occurred for at least some ~10-32 seconds or so, but it likely went on for longer. But how much longer? For seconds? Years? Billions of years? Or even an arbitrary, infinite amount of time? Has the Universe always been inflating? Did inflation have a beginning? Did it arise from a previous state that was around eternally? Or, perhaps, did all of space and time emerge from nothingness a finite amount of time ago? These are all possibilities, and yet the answer is untestable and elusive at present.