The baryon content of the Universe

The large discrepancy between the amount of baryons that were synthesized in Big-Bang and that we detect at $z=0$ locked in stars inside galaxies and in hot/cold gas in galaxies. goup and clusters, is a well known crucial issue for present day cosmology. It is also thought that this riddle is at the backbone of how the ordinary matter has formed, inside dark potential wells, the structures we see today. Then, it is useful to reprint, by means of this archive, the first (pre- astro-ph era) published work in which the amount of "luminous" baryons was actually and suitably computed and the disagreement with the BBN predictions strongly claimed. This because 12 years after the method and the results are still up-to-date, and above all, people are occasionally found to know only later works (appeared on astro-ph) that were meant to be just (ours or other's) follow ups of the present paper.