The biggest problem in theoretical physics today is the marriage between Quantum Mechanics and Gravity.  Throw in the fact that whatever theory comes out of it has to additionally be able to explain Dark Matter and Dark Energy, and we have ourselves a massive problem to solve.  How do we reconcile the seemingly random probabilistic nature of quantum mechanics with the smooth, pliable space-time of General Relativity.  We have two incredible theories that explain the Universe, make predictions accurately, and have led to amazing advances in technology and understanding, yet they completely disagree with each other at common scales. So...