Galaxies spin too fast for the matter we can see to hold them together.
In the Solar System, outer planets orbit more slowly than inner ones. Astronomers expected the same in spiral galaxies. Instead, Vera Rubin and Kent Ford found in the 1970s that outer stars move about as fast as inner ones. Something unseen, and massive, seems to be holding galaxies together.
A rotation curve plots how fast stars and gas orbit against their distance from a galaxy's centre. If most of a galaxy's mass sat in its bright central bulge, as its light suggests, the outer stars should move more slowly the farther out they are, just as the outer planets of the Solar System orbit more slowly than the inner ones. When astronomers measured real galaxies, the curves did not fall. Outside the centre they stayed roughly flat.
The clues built up slowly. Vesto Slipher first detected the Andromeda galaxy's rotation in 1914. In the 1930s Jan Oort and Fritz Zwicky suspected unseen mass in the Milky Way and in clusters of galaxies. Radio astronomers at the Dwingeloo observatory measured an extended rotation curve for Andromeda in 1957 that suggested mass spread far beyond the light, and in 1959 Louise Volders showed that the galaxy M33 also failed to spin as expected.
The decisive work came from Vera Rubin, using a sensitive new spectrograph with her colleague Kent Ford at the Carnegie Institution in Washington. In 1975 they announced that stars in spiral galaxies orbit at roughly the same speed across a huge range of distances, implying that a galaxy's mass keeps growing with radius well beyond most of its stars. Her influential 1980 paper suggested that either gravity does not behave as expected on galactic scales, or at least half of a galaxy's mass lies in a dark halo. The results met scepticism at first but have since been confirmed many times.
The favoured explanation is dark matter, invisible material that does not emit light but does exert gravity, and many other lines of evidence now support it. It is not unanimous, however. The best-known alternative, modified Newtonian dynamics or MOND, proposes instead that gravity itself works differently at very low accelerations. What dark matter actually is remains one of the biggest open questions in physics.
Source: Wikipedia — Galaxy rotation curve · Text summarised from Wikipedia (CC BY-SA 4.0)
