One of the most vexing problems in cosmology today is the nature of the relative distributions of mass and light. That these might be substantially different is an idea which, virtually unknown twenty years ago, has rapidly become established as almost received truth. This remarkable transformation is due to several developments in physics and astronomy. Dynamical studies of galaxies and groups of galaxies have shown that the ratio of luminosity density to mass density is not that expected for stellar populations, and not a universal constant. Recent theories of very high energy physics have suggested the existence of forms of stable matter other than those which participate in star formation. Finally, it has become clear that theoretical attempts to understand the structure of the universe will be much more successful if the structure is rather different than that suggested by the distribution of galaxies.
The consequences for astrophysics are profound. At least 95 percent of the matter in the universe is dark. It is possible that as much as 99 percent may be dark, and composed of particles very different than the baryons which constitute the solar system and the galaxy. If the latter alternative is correct, there will also be significant consequences for the practice of astronomy. As astronomers, we are totally dependent on observations of the luminous material for all information about the distribution, motions, and properties of the matter comprising the universe. If that luminous material is a minor constituent of the universe, unrepresentative of the properties of the dominant forms of matter, the tenuous links which connect the earth-bound observer with the rest of the universe will be greatly weakened.
Regrettably, the properties of the cosmos have not been arranged for the convenience of observational astronomers, and this radical alternative may be correct, even if it is troublesome. Because the interpretation of many observations, and the confrontation of those observations with theory will be fundamentally different if this hypothesis is correct, a speedy determination of its validity is of great importance for the progress of extragalactic astronomy. In this review, I shall discuss the genesis of this idea, its consequences for astrophysics, and what existing observations may say about its cortectness, with particular emphasis on scales larger than that of individual galaxies. Since masses, luminosities, and their ratio all depend on the assumed value of the Hubble Constant, H0, I shall, where convenient, express quantities in terms of h = H0 / 100 km s-1, to make the dependence explicit. Where that is not convenient, I shall assume H0 = 50 km s-1 Mpc-1.