What does the flat rotation curve of spiral galaxies suggest about dark matter?

The flat rotation curves of spiral galaxies indicate that the rotation speed of stars and gas remains constant at large distances from the galactic center, rather than decreasing as would be expected based on the visible matter alone. This observation suggests that there is a significant amount of mass that is not accounted for by the luminous matter (stars, gas, etc.) we observe.

The implication of this flat rotation curve is that dark matter must be distributed throughout the galaxy rather than being concentrated primarily in the center or only affecting smaller galaxies. The presence of dark matter throughout the galaxy provides the necessary gravitational influence to maintain the observed rotation speeds, ensuring that stars at the edges of galaxies move as quickly as those closer to the center. This comprehensive distribution aligns with our understanding of dark matter’s role in the universe, further supporting the idea that it is not limited in its effects to just the core or to specific types of galaxies.