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Jaan Einasto

Tartu Observatory, Estonia

Abstract. A review of the development of the concept of dark matter is given. The dark matter story passed through several stages on its way from a minor observational puzzle to a major challenge for theory of elementary particles.

I begin the review with the description of the discovery of the mass paradox in our Galaxy and in clusters of galaxies. First hints of the problem appeared already in 1930s and later more observational arguments were brought up, but the issue of the mass paradox was mostly ignored by the astronomical community as a whole. In mid 1970s the amount of observational data was sufficient to suggest the presence of a massive and invisible population around galaxies and in clusters of galaxies. The nature of the dark population was not clear at that time, but the hypotheses of stellar as well as of gaseous nature of the new population had serious difficulties. These difficulties disappeared when non-baryonic nature of dark matter was suggested in early 1980s.

The final break through came in recent years. The systematic progress in the studies of the structure of the galaxies, the studies of the large scale structure based on galaxy surveys, the analysis of the structure formation after Big Bang, the chemical evolution of the Universe including the primordial nucleosynthesis, as well as observations of the microwave background showed practically beyond any doubt that the Universe actually contains more dark matter than baryonic matter! In addition to the presence of Dark Matter, recent observations suggest the presence of Dark Energy, which together with Dark Matter and ordinary baryonic matter makes the total matter/energy density of the Universe equal to the critical cosmological density. Both Dark Matter and Dark Energy are the greatest challenges for modern physics since their nature is unknown.

There are various hypothesis as for the nature of the dark matter particles, and generally some form of weakly interactive massive particles (WIMPs) are strongly favored. These particles would form a relatively cold medium thus named Cold Dark Matter (CDM). The realization that we do not know the nature of basic constituents of the Universe is a scientific revolution difficult to comprehend, and the plan to hunt for the dark matter particles is one of the most fascinating challenges for the future.

Key Words: Dark matter, galaxies, clusters of galaxies, large-scale structure of the Universe

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