Question: What is quantum gravity?

Lewis Wright answered on 13 Mar 2017:

There are four fundamental forces: electromagnetism, the strong force, the weak force, and gravity.

The first three can be combined together in the early universe, and the nature of how they interact with each other is known.
Gravity is the odd one out, as it acts over theoretically infinite distances and is proportional the the amount of mass you have.

The problem is linking gravity, which is a long-range force, with the others, which are short-range. Quantum gravity is about trying to explain gravity through quantum mechanics, which is gravity on the smallest scales.