Theoretical Astrophysics is the discipline that seeks to explain the phenomena observed by astronomers in physical terms. With this purpose, theoretical astrophysicists create and evaluate models to reproduce and predict the observations. Theoretical astrophysicists use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving you insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that you would not otherwise see.
Theorists in astrophysics endeavor to create the simplest models possible that are in agreement with observations (from astronomers). Their models/theories must make testable predictions. New data can then be gathered which may be consistent or inconsistent with the theory; in the case of an inconsistency, the model may be discarded or (as is often the case) new complications are added to it.
Some widely-accepted theories/models in astrophysics include the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics. For an example of an astrophysical theory (although one which is not generally accepted by the astrophysical community), see the article Modified Newtonian Dynamics.