The baryon content of the universe

We estimate the baryon mass density of the Universe due to the stars in galaxies and the hot gas in clusters and groups of galaxies. The galaxy contribution is computed by using the Efstathiou, Ellis & Peterson luminosity function, together with van der Marel and Persic & Salucci's mass-to-light versus luminosity relationships. We find {OMEGA}_b_ ~ 0.002. For clusters and groups we use the Edge et al. X-ray luminosity function, and Edge & Stewart and Kriss, Cioffi & Canizares' (gas mass)-luminosity relations. We find {OMEGA}^gas^_b_~0.001. The total amount of visible baryons is then {OMEGA}_b_~0.003, i.e. less than 10 per cent of the lower limit predicted by standard primordial nucleosynthesis, implying that the great majority of baryons in the Universe are unseen.