Material Content of the Universe: Introductory Survey

Matter in the Universe can be detected either by the radiation it emits or by its gravitational influence. There is a strong suggestion that the Universe contains substantial hidden matter, mass without corresponding light. There are also arguments from elementary particle physics that the Universe should have closure density, which would also imply hidden mass. Observations of the chemical composition of the Universe interpreted in terms of the hot Big Bang cosmological theory suggest that this hidden matter cannot all be of baryonic form but must consist of weakly interacting elementary particles. A combination of observations and theoretical ideas about the origin of large-scale structure may demand that these particles are of a type which is not yet definitely known to exist.