Well, nothing, really. That’s the point. But many are unhappy with the philosophical implications of that:
Let’s face it: to think that the universe has a history that started with a kind of birthday some 13.8 billion years ago is weird. It resonates with many religious narratives that posit that the cosmos was created by divine intervention, although science has nothing to say about that.
If everything that happens can be attributed to a cause, what caused the universe? To deal with the very tough question of the First Cause, religious creation myths use what cultural anthropologists sometimes call a “Positive Being,” a supernatural entity. Since time itself had a beginning at some point in the distant past, that First Cause had to be special: it had to be an uncaused cause, a cause that just happened, with nothing preceding it.
Marcelo Gleiser, “What happened before the Big Bang?” at BigThink (June 9, 2021)
Gleiser goes on at some length, clarifying that science has nothing to say about events before the Big Bang: “The mystery of the First Cause remains. You can choose religious faith as an answer, or you can choose to believe science will conquer it all.”
But, in the context, what does “science will conquer it all” mean? How does science “conquer” the question: How did the universe come to exist? Either God or no answer.
What happened before anything happened? It’s a meaningless question within itself unless one posits a First Cause or God.
Science, like an afghan, tends to fray at the edges.
See also: The Big Bang: Put simply, the facts are wrong.