For the past century there have been two incompatible theories on how the universe works:

Gravity (courtesy of Einstein’s general theory of relativity) and
Quantum Theory (used for the other three fundamental forces of nature: electromagnetism and the strong and weak nuclear forces ).

Both work ok, except in extreme environments (like inside a black hole). So, scientists are looking for a good theory of Quantum Gravity. The problem is that relativity wants gravity to be continuous, while quantum theory wants it grainy.