Astronomers have been fascinated by galaxies for centuries. First came the realization that our solar system lay swaddled within the arms of a massive body of stars. Then came evidence that other galaxies existed beyond the Milky Way. By the 1920s, scientists like Edwin Hubble were cataloging thousands of "island universes" and recording information about their sizes, rotations and distances from Earth.
One key aspect astronomers hoped to measure was the mass of a galaxy. But you can't just weigh something the size of a galaxy – you have to find its mass by other methods. One method is to measure the light intensity, or luminosity. The more luminous a galaxy, the more mass it possesses (see How Stars Work). Another approach is to calculate the rotation of a galaxy's body, or disk, by tracking how quickly stars within the galaxy move around its center. Variations in rotational velocity should indicate regions of varying gravity and therefore mass.
When astronomers began measuring the rotations of spiral galaxies in the 1950s and '60s, they made a puzzling discovery. They expected to see stars near a galaxy's center, where the visible matter is more concentrated, move faster than stars at the edge. What they saw instead was that stars at the edge of a galaxy had the same rotational velocity as stars near the center. Astronomers observed this first with the Milky Way, and then, in the 1970s, Vera Rubin confirmed the phenomenon when she made detailed quantitative measurements of stars in several other galaxies, including Andromeda (M31).
The implication of all of these results pointed to two possibilities: Something was fundamentally wrong with our understanding of gravity and rotation, which seemed unlikely given that Newton's laws had withstood many tests for centuries. Or, more likely, galaxies and galactic clusters must contain an invisible form of matter – hello, dark matter – responsible for the observed gravitational effects. As astronomers focused their attention on dark matter, they began to collect additional evidence of its existence.