Scientists have studied the velocity dispersion patterns in the stellar populations near our sun, measuring the relative speed of travel in each of the stars and their surrounding celestial bodies.  By analyzing the patterns of velocity distribution among the observed bodies,  they are able to determine that there is twice as much material present within a few hundred light years of the sun than we can identify in luminous stars; this is also true in other nearby spiral galaxies.  This unknown, non-luminous matter, has been coined, "Dark Matter" by the scientists that study it.