Why do the rotation speeds of stars in galaxies suggest the presence of dark matter?

The rotation speeds of stars in galaxies provide compelling evidence for the existence of dark matter because they contradict predictions based on visible matter alone.1

In a system where mass is concentrated toward the center, such as our solar system, the orbital velocities of objects should decrease at greater distances from the center, following Keplerian motion. Astronomers expected galaxies to behave similarly, with rotational velocities dropping in the outer regions where visible starlight—and thus inferred luminous mass—is sparse.21

However, observations by astronomers such as Vera Rubin in the 1970s revealed that rotational velocities remain nearly constant even at large radial distances from the galactic center. This "flat" rotation curve indicates that there is significantly more mass present in the outer reaches of galaxies than can be accounted for by visible stars and gas. Because this additional material remains undetected across the electromagnetic spectrum, it is termed dark matter, and it is believed to form massive haloes that extend well beyond the visible edges of galaxies.12