Particle physics
The Standard Model is known to have problems. For example, it explains three of the four forces very well, but it cannot explain gravity. This is why general relativity, a different theory that explains where gravity comes from, needs to be included for physicists to explain the universe. There is a lot of work to improve the theory and/or find a better theory that is being done. This work is often called theoretical particle physics because it is about building better theories of particles. Theoretical particle physicists make theories to try to improve the Standard Model. One example of this is how many theories predict undiscovered particles.