You're right. The Big Bang theory really only concerns itself with what happens starting at around 10^−43 seconds after the presumed singularity that started the universe. That presumed singularity is just a consequence of running General Relativity backwards, and we know that quantum effects are also important at these kinds of insane energy densities, so the singularity can be thought of as a placeholder, until something better comes along. (A complete theory of Quantum Gravity would help a lot.)