Theoretical physics is a branch of physics that uses mathematical models and abstractions to explain and predict natural phenomena. It seeks to understand the fundamental principles governing the universe, such as the behavior of particles, forces, and energy. Theoretical physicists often develop theories that can later be tested through experiments. Key areas of theoretical physics include quantum mechanics, which studies the behavior of very small particles, and general relativity, which describes the gravitational force and the structure of space-time. The work of theorists like Albert Einstein and Niels Bohr has significantly advanced our understanding of the physical world.