Dark matter. Abstract illustration of dark matter showing a spiral galaxy as a jigsaw puzzle with many of the pieces missing. Dark matter is a theoretical concept used by cosmologists to explain why the stars in a galaxy stay together. The gravitational force within a galaxy can be determined by its rotational period. The mass of the stars can be calculated by measuring their luminosity. Cosmologists estimate that only about 10% of the gravitational force holding galaxies together comes from visible mass. Theoreticians have suggested that the other 90% comes from dark matter made of subatomic particles or compact, non-luminous material such as asteroids.