Dark Baryons in the Universe: the Quest Goes On
  We show that the high local baryonic fraction, M(bar) ~ 1/3 M(tot), found in groups and clusters of galaxies does not reconcile the observed cosmological baryon density with the standard Big-Bang prediction. Taking into account recent measurements on the hot-gas content and temperature functions of clusters and groups, we get Omega(gas) = 0.0023 h_{50}^{-1.5} ~ 4% Omega(BBN). Including the contributions of galaxies and of (local) Ly-alpha clouds we estimate Omega(bar) ~ 0.004-0.006 < 10% Omega(BBN) as the amount of detected baryons. Most of the synthesised atoms are still to be discovered. We propose to relate the impressive presence of the hot gas component in clusters with the very low, mass-dependent efficiency of the process of galaxy formation in making stars from the primordial gas. 
Paolo Salucci and Massimo Persic (SISSA and Osservatorio Astronomico,   Trieste)