Dark matter composition research - Ordinary matter

Scientists turned initially to ordinary matter (baryon) for their research and reviewed all the types of particles which could contribute to this gravitational field, such as gas clouds, dead stars or black holes.

Gas clouds?

In the 1990's, precise cartographies of the universe x-rays emission sources - gathered thanks to the Rose Satellite - highlighted the presence of gigantic ionized gas clouds within galaxy clusters; clouds of several million degrees non-emitting in the visible field. Moreover, these clouds seemed to contain ten times more matter (at least, luminous matter) than the galaxies of these clusters. Was this finally the missing matter?

Unfortunately not. On the contrary, these clouds are the proof of the presence of dark matter around the galaxies. Indeed, to reach such temperatures, the particles constituting the cloud must be accelerated at very high speeds (approximately 300 km/s), and this acceleration comes from the force of gravitation. However the quantity of gas is insufficient to generate such a gravity field. Similarly, the stars alone cannot prevent the gas cloud from escaping. The gravitational influence of the dark matter is then necessary to explain the containment of these clouds near the galaxies. Moreover, the shape of these clouds are helping the astronomers and astrophysicist in their research to determine the dark matter's distribution in the neighbourhood.

Black holes?

Much more massive than MACHO or stars, black holes could have been good candidates. Some of them reach a mass of 10.000 solar masses (in particular supermassifs black holes, in galaxy's centers). However, it would be necessary to have nearly a million of such black holes in a galaxy to fill the lack of matter; a too large number knowing the effect of black holes on neighbouring stars. Indeed, black holes sometime cross the galactic disc and disturb the movement of stars. With such a number of black holes, the movements of these stars would be strongly amplified, which would make the galactic disk thicker than what is currently observed.

Remain the stellar black holes (a few solar masses), not easily detectable, and the black holes of a few tens or hundreds of solar masses, whose nature of formation has not been explained yet. In all cases, the track of black holes as being the famous dark matter was abandonned, and the astronomers are leaning on the nonbaryonic matter type.

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Baryonic Nonbaryonic Neutrino WIMP String theory