  A short story of dark matter as an example of a scientific revolution is
given. The characteristics of stellar populations are compared with those of
dark halos. The mean mass-to-luminosity ratio of stellar populations is found
to be 4 in Solar units. Dark matter around galaxies, and in groups, clusters
and voids is discussed. Modern data suggest that the overall density of matter
in the Universe is 0.3 +- 0.1, about 80% of this matter is non-baryonic dark
matter, and about 20% is baryonic, mostly in the form of hot intra-cluster and
intragroup gas, the rest in stellar populations of galaxies. The Universe is
dominated by dark energy (cosmological constant) term.
