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People often ask: Who discovered dark matter? The dark matter story is a typical scientific revolution (Kuhn 1970, Tremaine 1987). As often in a paradigm shift, there is no single discovery, the new concept was developed step-by-step.

First of all, actually there are two dark matter problems - the local dark matter close to the plane of our Galaxy, and the global dark matter surrounding galaxies and clusters of galaxies. The milestones of the local dark matter problem solution are the studies by Öpik, Oort, Kuzmin, Bahcall and Gilmore. Dark matter in the Galactic disk, if present, must be baryonic (faint stars or jupiters). The amount of local dark matter is low, it depends on the boundary between luminous stars and faint invisible stars.

The story of the global dark matter also spans many decades. It began with the work by Zwicky (1933) on the Coma cluster, was continued with the study by Kahn and Woltjer (1959) on the dynamics of the Galaxy-M31 system, and statistical determinations of masses and mass-to-luminosity ratios of pairs, groups and clusters of galaxies. For some reason, these studies did not awake the attention of the astronomical community. However, the awareness of the presence of a controversy with masses of galaxies and galaxy systems slowly increased.

Further development of the dark matter concept was influenced by the East-West controversy during the Cold War (on this controversy see Fairall 1998, p. 11 - 12). The dark matter puzzle was solved in 1974 by two independent studies of masses of galaxies by Tartu and Princeton astronomers. It was suggested that all giant galaxies are surrounded by massive halos (coronas), and that dark matter is dynamically dominant in the Universe. As usual in scientific revolutions, the general awareness of a crisis comes when the most eminent scientists in the field begin to concentrate on the problem. This happened when the Princeton group, Burbidge (1975) and Materne & Tammann (1976) published their contributions pro and contra the dark matter hypothesis. In the following years experimenters devoted themselves to finding new evidence in favour of (or against) the new paradigm. The work by Rubin and collaborators on galaxy rotation curves, our work on properties of satellite systems of galaxies and the Magellanic stream, X-ray studies of clusters, as well as investigation of gravitational lensing in clusters belong to this type of studies.

The word on the crisis spread more rapidly in the East: the first dark matter conference was held in Tallinn in 1975, the first official IAU dark matter conference was held only ten years later. The first popular discussions of the problem were given in "Priroda" and "Zemlya i Vselennaya" (the Russian counterparts of "Scientific American" and "Sky & Telescope") by Einasto (1975) and Einasto, Chernin & Jõeveer (1975), and also in the respective journal in Estonian. In USA the first popular discussions were given many years later (Bok 1981, Rubin 1983). However, most experimental studies confirming the dark matter hypothesis were made by US astronomers, and the cold dark matter concept was also suggested by Western astronomers.

The new paradigm wins when its theoretical foundation is established. In the case of the dark matter this was done by Blumenthal et al. (1984) with the non-baryonic cold dark matter hypothesis. Also the need for non-baryonic dark matter was clarified: otherwise the main constituents of the universe - galaxies, clusters and filamentary superclusters - cannot form.

In the following years main attention was devoted to detailed elaboration of the concept of the cold dark matter dominated Universe. Here a central issue was the amount of dark matter. Initially opinions varied from a moderate density of the order of 0.2 critical density up to the critical density. Only a few years ago it was clarified that dark matter constitutes only 0.25 of the critical density, and the rest is mostly dark energy, characterized by the cosmological constant or the OmegaLambda-term.

To conclude we can say that the story of dark matter is not over yet - we still do not know of what non-baryonic particles the dark matter is made of.


I thank M. Jõeveer and E. Saar for fruitful collaboration which has lasted over 30 years. This study was supported by the Estonian Science Foundation grant 4695.

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