A bit speculative. Still an irrational idea rationally deduced but with some empirical data.
In the first fraction of a second, the universe cooled enough for the electromagnetic and weak forces to split, setting the stage for stable particles.
Physicists often talk about the idea that the universe exploded from nothing in a singularity, that idea is more philosophical than scientic. The universe’s expansion is scientific, the singularity itself remains speculative.