is the branch of astrophysics that uses the tools of theoretical physics to explain the motion and arrangement of stars, gas, and dark matter within galaxies. It primarily focuses on understanding how gravitational forces shape galactic structures—like spiral arms and central bulges—over billions of years. Core Theoretical Concepts

: Studying processes like dynamical friction (the "drag" on massive objects moving through a sea of stars) and violent relaxation . Essential Resources

: Explaining the origin and stability of spiral structures through density wave theory.