What Happened Before the Big Bang?
The Big Bang was the beginning of all the particles and matter we have today, but not the beginning of the universe itself. The big bang expansion was preceded and set up by a previous stage of the observable universe known as Cosmic Inflation. How do we know this? 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. Most physicists believe Cosmic Inflation is the reason. Cosmic Inflation theory has alredy passed 4 of the theory's 6 falsification tests.
Theoretically and observationally, the evidence for cosmic inflation preceding and setting up the Big Bang is incredibly strong and comprehensive...There are six major predictions of cosmic inflation that were definitively extracted before they were ever put to the test...We’ve since observed strong evidence favoring cosmic inflation for points 1, 3, 4, and 5
https://bigthink.com/starts-with-a-bang/why-big-bang-happened/
Here is a technical article describing in detail what Cosmic Inflation was like.
https://bigthink.com/starts-with-a-bang/when-cosmic-inflation-occurred/The Big Bang theory timeline starts at time t=0 but the hot big bang doesn’t occur until t=10-32 seconds. What occurred at time t=0? We have no reason to think any special event occured at t=0. It is simply the farthest time we can extrapolate backwards to. The time between t=0 and the hot big bang represents the final fraction of a second of the previous cosmic inflation phase of the universe, which had an unknown length that could possibly have lasted billions of years
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?...These are all possibilities, and yet the answer is untestable and elusive at present.
https://bigthink.com/starts-with-a-bang/universe-infinite/- All the particles and matter that we have today were created after the big bang event (hot big bang), which occurs at t=~10-32 seconds in the big bang timeline, from the preexisting energy of Cosmic Inflation.
- The big bang was preceded and set up by an earlier era of cosmic inflation. We don't know when cosmic inflation started or how long it lasted. But new space was created at a much faster rate in the earlier inflation than in the later expansion, so the big bang event involved a drastic slowdown in the rate of new space creation, allowing the formation of our present particles and matter.
- We have reason to suspect something else came before cosmic inflation.