Nobody knows. It's like knowing how electronics work, from transistors downwards; and how software works, from machine language upwards; but completely missing

Nobody knows. It's like knowing how electronics work, from transistors downwards; and how software works, from machine language upwards; but completely missing the entire concept of a CPU, how it's built, how complex (or not) it is, and how to study it and tinker with it. Quantum gravity is one of the biggest remaining mysteries or unknowns in Physics. (IANAP)