What are the four key components used to define a galaxy as a massive collection of matter?

Stars, gas, dust, and dark matter

The fundamental definition of a galaxy centers on it being a colossal, gravitationally bound entity composed of specific constituent elements. The text specifies that a galaxy is characterized by a massive collection of stars, which provide the visible light component, alongside significant amounts of gas and dust, which serve as raw material for future stars. Crucially, the structure is held together by gravity, much of which is attributed to the presence of dark matter, which is incorporated into the standard definition alongside the baryonic components listed. These four elements—stars, gas, dust, and dark matter—are held together by mutual gravitational attraction.