Results of these calculations were reported at the First European Astronomy Meeting by Einasto (1974). The main conclusion was: it is impossible to reproduce the rotation data by known stellar populations only. The only way to eliminate the conflict between photometric and rotational data was to assume the presence of an unknown almost spherical population with a very high value of the mass-to-light ratio, large radius and mass. To avoid confusion with the conventional stellar halo, the term "corona" was suggested for the massive population. Thus, the detailed modelling confirmed earlier results obtained by simpler models. But here we have one serious difficulty - no known stellar population has so large a M / L value.
Additional arguments for the presence of a spherical massive population in spiral galaxies came from the stability criteria against bar formation, suggested by Ostriker & Peebles (1973). Their numerical calculations demonstrated that flat systems become rapidly thicker and evolve to a bar-like body. In real spiral galaxies a thin population exists, and it has no bar-like form. To remain stable galaxies must have a massive spherical halo.
The rotation data available in the early 1970s allowed the determination the mass distribution in galaxies up to their visible edges. In order to find how large and massive galactic coronas or halos are, more distant test particles are needed. If halos are large enough, then in pairs of galaxies the companion galaxies are located inside the halo, and their relative velocities can be used instead of the galaxy rotation velocities to find the distribution of mass around giant galaxies. This test was made by Einasto et al. (1974). A similar study was made independently by Ostriker et al. (1974). Our results were first discussed in the Caucasus Winter School on Cosmology in January 1974 and in the Tallinn Conference on Dark Matter in January 1975 (Doroshkevich et al. 1975).
The mass of galactic coronas exceeds the mass of populations of known stars by one order of magnitude. According to new estimates the total mass density of matter in galaxies is 20% of the critical cosmological density. The data suggest that all giant galaxies have massive halos/coronas, thus dark matter must be the dynamically dominating population in the whole Universe.
Initially the presence of massive coronas/halos was met with scepticism. In the Third European Astronomical Meeting the principal discussion was between the supporters of the classical paradigm with conventional mass estimates of galaxies, and of the new one with dark matter. The major arguments supporting the classical paradigm were summarised by Materne & Tammann (1976). Their most serious argument was:
"Big Bang nucleosynthesis suggests a low-density
Universe with the density parameter
0.05; the
smoothness of the Hubble flow also favours a low-density Universe."
Additional observational data gave strong support to the presence of massive coronas/halos. Available rotation data were summarised by Roberts (1975). Extended rotation curves were available for 14 galaxies. In all galaxies the local mass-to-light ratio in the periphery reached values over 100 in solar units. Rubin et al. (1978, 1980) measured optically the rotation curves of galaxies at very large galactocentric distances. Bosma (1978) measured rotation data for 25 spiral galaxies with the Westerbork Synthesis Radio Telescope. Both results suggested that practically all spiral galaxies have extended flat rotation curves.
Another very important measurement was made by Faber & Jackson (1976), Faber et al. (1977), Faber & Gallagher (1979). They measured the central velocity dispersions for 25 elliptical galaxies and the rotation velocity of the Sombrero galaxy, just outside the main body of the bulge. Their data yielded for the bulge of the Sombrero galaxy a mass-to-light ratio M/L = 3, and for the mean mass-to-light ratios for elliptical galaxies about 7. These results showed that the mass-to-light ratios of stellar populations in spiral and elliptical galaxies are similar for a given colour, and the ratios are much lower than accepted in earlier studies based on the dynamics of groups and clusters. In other words, high mass-to-light ratios of groups and clusters of galaxies cannot be explained by visible galactic populations.
The distribution of the mass in clusters can be determined if the density and the temperature of the intra-cluster gas are known. These data can be measured by the Einstein X-ray orbiting observatory. The mass of Coma, Perseus and Virgo clusters was calculated from X-ray data by Bahcall & Sarazin (1977), Mathews (1978). The results confirmed previous estimates of masses made with the virial method using galaxies as test particles.
Finally, masses of clusters of galaxies can be measured using gravitational lensing of distant galaxies by clusters. The masses of clusters of galaxies determined using this method, confirm the results obtained by the virial theorem and the X-ray data (Fischer & Tyson 1997, Fischer et al. 1997).
Earlier suggestions on the presence of mass discrepancy in galaxies and galaxy systems had been ignored by the astronomical community. This time new results were taken seriously. However, it was still not clear how to explain the controversy of the Big Bang nucleosynthesis and the smoothness of the Hubble flow, discussed by Materne & Tammann (1976).