What is the primary conceptual difficulty the Big Bang Theory has in describing the absolute beginning?

Answer

The physics breaks down when trying to model the singularity itself.

The theory details evolution *after* the initial state, but General Relativity fails at the infinitely high density and temperature of the initial singularity, requiring a theory of quantum gravity.

What is the primary conceptual difficulty the Big Bang Theory has in describing the absolute beginning?
Origintheoryuniversecosmology