Observations of visible matter, the only kind we can see directly, suggest that most of the universe is, in fact, composed of dark matter. This conclusion comes mainly from the belief that something unseen (dark matter) is tugging on visible matter, making it do things the laws of motion say it should not do. All visible bodies, therefore, seem to be careening about in a dense cloud of unseen, unknown masses. These might be dark, Jupiter-sized objects, black holes, and/or some exotic forms of matter. We must choose between the reality of dark matter or admit that something is awry with our laws of gravitation and motion when they are applied on a cosmological scale.
The dark matter surrounding a galaxy will, according to the Theory of Relativity, act as a gravitational lens that will deflect light rays passing near it. This dark matter, acting like a telescope, should increase the number of quasars counted in the sky near galactic clusters. Such larger quasar counts are indeed observed, but these increases are much larger than expected. The implication is that there is much more dark matter in the universe than previously thought. (Ron Cowen, "Quasar Count Poses Dark-Matter Puzzle" Science News, 143:397, 1993.)