Baryonic dark matter

Recent determinatios of baryonic density using the angular power spectrum of the Cosmic Microwave Background are very close to the classical estimate from Big Bang Nucleosynthesis. This reinforces the case for dark baryons in the Universe and for a large component of exotic cold dark matter. Present-day baryons can be hidden in substellar objects, stellar remnants, cold gas clouds, hot diffuse ionized gas in various astrophysical environments. Direct detection searches and microlensing experiments provide estimates of the Galactic mass budget in massive compact objects concluding that the bulk of the dark matter in the halo of the Galaxy cannot be associated to MACHOs. Baryons in high redhshift Lyman-alpha systems can account for the cosmic baryonic density. However, the dominant form of present-day baryons and, in particular, the nature of the halo dark matter remains a mistery.