The main evidence for dark matter comes from rotation curves of disk galaxies. A rotation curve measures the rate at which stars and/or gas move in their circular orbits around the center of the galaxy. In general the curves are "flat", that is, the stars and gas rotate at the same speed from the inner parts out to the edge. This is not at all what one expects from he visible matter in the galaxies --- the outer parts of the galaxy should move slower than the inner parts, like the planets do in the Solar System. At Tuorla Observatory, Chris Flynn and  Rami Rekola are examing faint spiral galaxies to see if they also contain dark matter.