honestly? nobody knows, this is right at the edge of what physics can even ask. some think it's eternal inflation (endless bubble universes), some think it's a "big bounce" from a previous universe collapsing. and some physicists will just tell you the question doesn't even make sense if time itself started at the bang.
vikram.singh appreciate the detailed answer!
andrew_shaw wait really? that's crazy
quantum cosmology models try to smooth this out with stuff like the "no-boundary proposal" from Hawking and Hartle, basically saying time itself gradually "fades in" rather than starting at a sharp point.
some physicists just say "the question is like asking what's north of the north pole", if time started at the bang, "before" might not even be a coherent concept.