How do measurements of galaxy rotation provide evidence for dark matter?

Radio telescopes provide essential evidence for dark matter by allowing astronomers to map the rotation of galaxies through the observation of neutral hydrogen gas.12

By measuring the Doppler effect in the 21-cm spectral line of atomic hydrogen, astronomers can determine the rotational velocities of gas clouds at varying distances from a galaxy's center. In a system where mass is concentrated primarily in the visible center, orbital velocities should decrease at greater distances, following Keplerian motion.21

However, observations consistently show that these rotational velocities remain constant or even increase slightly at the outer edges of galaxies. This indicates that the gravitational pull is much stronger than what can be accounted for by the visible starlight and gas alone. Because this rapid rotation persists at distances where no luminous matter is detected, it implies the presence of a vast, invisible component—dark matter—that provides the necessary additional mass to hold the galaxy together.132