I don't necessarily subscribe in full to the theory, but there's a difference between the definition of "theory" and the definition of "scientific theory", the former being pretty much a guess, and the latter being a working model of a scientific concept. That said, the evidence is strongly in favor of it.
They know the universe is expanding, and that it's speeding up. When they do the math, it works out to everything originating from a singularity smaller than an atom and infinitely hot.
This is considered a fact by the vast majority of astronomers, astrophysicists, and quantum physicists.
I personally don't know why it has to be so small, except that it would require that much energy to cause such an expansion.