... Mysterious motions of galaxies . Invisible matter? ... Other ways to "see" dark matter . ... Most of a galaxy is not stars and gas, it is dark matter. ... Dark matter affects the motion of the galaxies moving inside a cluster of galaxies, and the motion of the clusters relative to each other. ... Dark matter also provides the seeds for galaxy formation. ... Components of Invisible Galaxies: Dark Matter & Galaxies , an interactive exhibit developed by the authors as part of Seeing is Believing . ...