quantum theory

I would start at Wikipedia.

Loosely, quantum theory is the study of the "micro-universe" of the atom, its constituent components, and the forces involved with their makeup.

This excludes gravity, per se, but there are efforts to find a grand unification of all forces, including gravity, at a quantum level. However, at a quantum level gravity really doesn't play a part. The rub is that quantum theory and physics outside the domain of quantum mechanics remain two discrete entities.

To exactly describe quantum theory would take a lifetime, so good luck with your career!

As to what the relation is with the creation of the universe, well, at the earliest moments of the Big Bang the conditions were ruled by the laws of quantum mechanics and not by the rules of classical physics because there were no atoms yet, just sub atomic particles and a whole lot of energy. Understanding quantum mechanics and the conditions at the early moments of the Big Bang are essentially the same.

But they don't know what gravity is or if it would act differently on these particals.

Don't trust any scientist.

There's a neat little book Quantum Physics by John Gribbin which explains the overall picture pretty well. It's $7 US from DK Publishing (I think Amazon or Borders should have it).