First observational evidence of dark matter

It is only 40 years later, in 1970, that the question of the existence of this dark matter reappeared. Starting from the analysis of the spectra of galaxies, the American astronomer Vera Rubin studied the rotation of spiral galaxies. The problem was the same as the comparison between the dynamic and the luminous mass of the galaxy clusters. It was a question of knowing if "luminous mass", i.e. the mass which is calculated from the presence of stars, is relatively equal (except for some corrections) to the dynamic mass.

It should be noted that the dynamic mass is normally the only true mass, as it is a measurement of the mass deduced from its gravitational influence. Any mass being subjected to gravitation, there is no reason to think that the dynamic mass observed is false. It is not as simple for the luminous mass. To measure the latter, the assumption is made that all the mass of the galaxy (or the galaxy cluster) is made of stars. These stars radiate, and if one knows (but it is very difficult) their distribution (mass, number, age, etc), the visible light is then a good way to determine the mass.

By analyzing the spiral galaxies' spectrum such as the Andromeda Galaxy (which section is visible), it is possible to calculate the curve from its rotation. The curve of rotation describes the number of revolutions of the galaxy according to the distance to the center. This curve of rotation is a direct measurement of the total distribution of matter in the galaxy. The maximum speed of rotation of a spiral galaxy is located at a distance of a few kiloparsecs from the center. It is then supposed to decrease, following a Keplerian decrease. Indeed, the stars at the periphery of the galaxy are in orbit around the center (in the same way as planets orbit around the Sun). The stars in periphery of the galaxy thus turn less quickly than those closer to the center. The curve of rotation, after a maximum, starts to go decrease again.

However, Vera Rubin observed that the stars located at the periphery of the Andromeda Galaxy - as for other spiral galaxies - appeared to rotate too fast (speed remained almost constant when the distance to the center increased). The curve of rotation of spiral galaxies (some of them) was flat. Speed did not decrease whereas one moved away from the center. Many other similar observations were carried out in the 1980s, reinforcing those of Vera Rubin. These observations raised deep questions, because the curve of rotation is a good measure of the dynamic mass. No assumption about the age or the stars' mass distribution is necessary

A possible explanation was to think of the existence of a huge nonvisible matter halo surrounding the galaxies; a halo which would represent up to 90% of the galaxy's total mass. Thus all the stars are almost in the center of the true extension of the "galaxy" (this time made up of the visible galaxy and the matter halo), and thus rotate normally. In other words, the stars located at the visible periphery of the galaxy, are not "far enough" from the center to decrease the curve of rotation. It still remains to directly observe this famous matter to confirm that it is the right explanation.

The presence of dark matter is one of the possible explanations. It has the immense advantage of being simple and of going in the good direction. Indeed, the astronomers suspected that galaxies contain non-luminous stars (such as dwarf brown, dwarf white, black holes, neutron stars) which could represent a large part of the total mass of the galaxy, but which is not visible with optical instruments. The observation of spiral galaxies in other wavelengths (in order to better characterize the presence of not very luminous objects in the visible field) was one of the major efforts of astronomy to research and understand the problem.

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Introduction First observational evidences The galactic rotation problem
Dark matter within galaxies Dark matter between galaxies Dark matter Composition
Baryonic Nonbaryonic Neutrino WIMP String theory