The energy spectrum of the universe

The energy and mass densities of the various observed and unobserved constituents of the universe are discussed. Consideration is given to the mass densities of the observed galaxies and possible unobserved galactic haloes, the cold diffuse intergalactic gas as estimated from quasar absorption lines and hot intergalactic gas detected as X-ray emission, and to the energy densities of the diffuse radiation fields at frequencies from the radio to the hard gamma ray, galactic cosmic rays and extragalactic relativistic electrons and nuclei, and as yet unobserved gravitational radiation and neutrinos. It is noted that most components of mass and energy in the universe probably arise from the discrete radiating objects (galaxies) and the microwave background radiation by the conversion of thermonuclear or gravitational energy or through interactions between the microwave photons and the components of discrete sources