There are a ton of competing models for how the early universe formed. In order to explain why the universe is so smooth and flat though, they all invoke the idea of a short (10e-37 seconds) period of time immediately following "the singularity" that is presumed to have been literally the first point. During inflation the universe blows up 100000 times in size (and correspondingly drops in temperature by the same factor) then immediately slows down to roughly the rate of expansion we see today.
There are a lot of simulations and theories about this could have worked. And I'm sure they all have lots of grounding and math and believers. But none of thr explanations I've ever heard amount to more than "when I do this funny thing, the math works and none of of us know why" and that has been the state of quantum physics for 70 years: a series of "we don't know but the math works."
In software, we call that tech debt and I feel like our current model of profit-driven science isn't capable of actually finding or reporting the answers that underly the debt-riddled results out of modern labs.