Neither string theory nor M theory is required to explain the BB, as they are involved with reconciling quantum mechanics with other forces, like gravity.
A quantum fluctuation where is an easy enough question to answer, if there is nothing and a quantum fluctuation occurs so that for a brief period some energy, balanced by negative energy of an equal amount, comes into existence briefly, then a universe will be created.
http://www.astrosociety.org/pubs/mer...2/nothing.html
This summarises it succinctly.
I don't even know what theoretical physics has to do with atheism anyway, these ideas don't deal with religion, they just deal with astrology and physics.