The thing is.. we know the Big Bang happened because there's a MASSIVE ammount of evidence for it, but studying and figuring out what went on before it (or even what happened before the first five secconds) is pretty much impossible without gaining a LOT more knowledge. We first need to focus on unifying Quantum Mechanics with Relativity.. and then we have to figure out what all this Dark Energy is before we can even BEGIN to tackle the cause of the Big Bang.
There are lots of theories to the cause of the big bang, but they'll all be hard to prove without a grand unified theory under our belts.
Physics and Cosmology is a subject I read quite a bit about (About half of the bookcase in this room is full of books on the subjects). It's very interesting stuff.
