... Dark matter Next . Disk galaxies become progressively fainter as one moves outward in radius from the centre - there are lower densities of stars, gas and dust at large radii Next . Tracing the rotation curve - how fast the material is orbiting within the Galaxy - one would expect the velocity to decrease as the light decreases, and we run out of galaxy. ... Brown dwarfs can have extremely low luminosities - if they were present in sufficient numbers, they could supply this `missing mass'. ...