… The whole expanding universe was described by the equations of general relativity, Einstein's theory of space and time and gravity, but the singularity was the one place where the equations couldn't go. If general relativity provides a map of the universe, the singularity is the uncharted spot that the cartographers aren't sure how to draw. Here be dragons.

Quantum dragons, most likely. The singularity suggested that general relativity would eventually give way to a more fundamental theory, but physicists already knew that Einstein's theory wasn't compatible with quantum mechanics, the theory that describes the behavior of matter at extremely small scales. In their day-to-day lives physicists could ignore the problem by keeping the two theories separate, using general relativity to describe how big things such as planets and galaxies distort spacetime and using quantum mechanics to describe the strange dice game subatomic particles play. But at the end of the day, the separation can't hold up.