Wishing to avoid controversial areas of astronomy, including quasars and galactic motion, Rubin began to study the rotation and outer reaches of galaxies, an interest sparked by her collaboration with the Burbidges. She investigated the rotation curves of spiral galaxies, again beginning with Andromeda, by looking at their outermost material. She observed flat rotation curves: the outermost components of the galaxy were moving as quickly as those close to the center. This was an early indication that spiral galaxies were surrounded by dark matter haloes. She further uncovered the discrepancy between the predicted angular motion of galaxies based on the visible light and the observed motion. Her research showed that spiral galaxies rotate quickly enough that they should fly apart, if the gravity of their constituent stars was all that was holding them together; because they stay intact, a large amount of unseen mass must be holding them together, a conundrum that became known as the galaxy rotation problem.