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 ^ { stars } _ { b } \simeq 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 _ { b } ^ { gas } \simeq 0.001 .
The total amount of visible baryons is then \Omega _ { b } \simeq 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 .