First observational evidence of dark matter

In 1933, Swiss astronomer Fritz Zwicky of CalTech decided to study a small group of seven galaxies in the Coma Cluster. Its objective was to calculate the total mass of this cluster by studying the speed (or rather the dispersion speeds) of these seven galaxies. By using Newton laws, he calculated its mass 'dynamic mass', then compared it with the 'luminous mass', which is the mass calculated from the quantity of light emitted by the cluster (by making to the assumption of a raisonable distribution of the star population in the galaxies).

The dispersion speeds (or in other words, how the speed of these 7 galaxies differed from each other) is directly related to the cluster's mass. In fact, a star cluster can be compared with a gas, where the particles would be galaxies. If the gas is hot and light, the dispersion speed of the particles is high. In the extreme case, the particles which have a sufficient speed leave the gas (evaporation). If the gas is cold and heavy, the dispersion speed is weak.

Zwicky was surprised to note that the speeds observed in the Coma Cluster were very high. The dynamic mass was 400 times larger than the luminous mass! At the time, the methods and the precision of measurements were not accurate enough to be neglected. Moreover, massive objects such as brown dwarf, white dwarf, neutron stars , black holes and in general of poorly non radiating objects were little known. And same for interstellar dust and molecular gas.

Zwicky announced its observation to its fellows, but they were not interested. Zwicky's reputation was not so good due to a strong character and its measurements were criticized due to measurement uncertainties.

The same phenomenon was again observed in 1936 by Sinclair Smith during the calculation of the Virgo Cluster's total dynamic mass. This one was 200 times more important than Edwin Hubble's estimate. According to Smith, is could be explained by the presence of matter between the galaxies of the cluster. Moreover, the galaxy clusters were still considered by a great number of astronomers as of temporary structures rather than of stable structures. This explanation was enough to justify excessive speeds.

At the time, astronomers had other 'more imporant'questions to solve (such as the expansion of the Universe) and the question of this difference between the dynamic and luminous mass was let aside for several decades.

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