One of the fundamental problems in cosmology is to compile a census of the contents of the universe. Material at a range of different densities and temperatures can be detected by emission or absorption somewhere in the electromagnetic spectrum. Gravity, however, detects mass quite independently of its equation of state. In an ideal world, these two routes to the total density would coincide; in practice, the gravitational route is able to detect more mass by a factor of up to ten than can be detected in any other way. This chapter, and the two that follow, summarize some of the methods that have been used to learn about the gas, radiation, dark matter and galaxies that together make up the observed universe.

Background radiation

We start with the constraints on any large-scale distribution of matter. A near-uniform intergalactic medium (IGM) will manifest itself in ‘background’ radiation that is isotropic on the sky. In many wavebands, the background radiation originates at sufficiently large distances that we are seeing back to a time when there were no discrete objects in existence. However, in other wavebands, the background may consist of the contribution of a large number of discrete sources, which are too faint to be detected individually. Studying backgrounds of this type tells us about the integrated properties of galaxy populations at more recent times, which can also provide crucial cosmological information.