What role does dark matter play in galaxies?

Dark matter plays a crucial role in the structure and formation of galaxies due to its gravitational effects. Although it does not emit or absorb light, dark matter is thought to comprise a significant portion of the total mass in the universe. Its presence influences the motion of visible matter, such as stars and gas, within galaxies.

The gravitational pull exerted by dark matter helps to hold galaxies together and affects how they form and evolve over time. It creates a framework for visible matter to clump around, leading to the formation of galaxies and their structures. Dark matter's influence is particularly noticeable in the rotation curves of galaxies, where the rotational speeds of stars at different distances from the galactic center cannot be explained solely by the visible mass of stars and gas.

In contrast, the other options misrepresent the function of dark matter. Dark matter does not provide light, contribute to the visible mass in the sense of adding to the count of stars, nor does it serve as an energy source in the way we think of stars producing energy through nuclear fusion. Understanding dark matter's role helps clarify the dynamics of galaxies and the broader structure of the universe.