La Masse Manquante dans L'Univers

The first evidence for missing mass or dark matter comes from the 30's. On one hand, Oort noted that in the solar neigbourhood the mass of the stars (inderred from count numbers) cannot account for their observed velocities. On the other hand, observation on the sky of various galaxy condensations like the Coma Cluster let suppose that they are actual bound systems and not only statistical fluctuations. However, with such assumption, Zwicky concluded that the velocity dispersion of galaxies in Coma required 100 times more mass than contained in galaxies. Since this period, refined observations, analyses and a reevaluation of the cosmic distance scale reduced this factor but the problem is still present. It is particularly striking for spiral galaxies where systematic observations of rotation curves lead to infer the presence of spherical massive halos. These dynamical evidences form the "first missing mass problem". The " second one " appears with the development of Great Unified Theories for which the natural laboratory is the very early Universe. A consequence of these theories is that our Universe could be closed by "exotic" particles which interact only gravitationally . We shall try to shed some light on this dark matter asking for its content and nature.