The Biggest Unsolved Problem in Physics

95% of the universe is not understood at all. Various theories and models have been proposed, but none has been experimentally verified or falsified thus far. So there's a huge unsolved problem in theoretical physics. What are black holes? What is inside a black hole? Why do we have singularities? That needs to be solved. Quantum gravity. We don't understand gravity at all. General relativity is not compatible with quantum mechanics. In quantum mechanics, space is smooth and time is absolute. In general relativity, space is curved and time is relative. The completely different ways of looking at the world doesn't match - so these two don't match. But we need to make them match