The dark matter

At present, we still don’t know the real composition of our Universe! Researchers found several evidences about the existence of an invisible matter, able to connect with its gravitational attaction galaxies and cluster of galaxies. It constitutes about the 90% of the whole universal matter. Its secret nature is still misterious, anyway its presence is revealed by some indirect effects in the surrounding space:

  • Spiral galaxies rotation. If we observe the trend of rotation speed in a spiral galaxy, in operation to the distance of the galactic centre, we will discover that, in large distances, this speed remains elevated, instead to decrease. We can explicate this phenomenon just admitting the existence of other matter, not visible, able to produce a strong gravitational field, increasing the speed.
  • Speed distribution in the galactic clusters.A single cluster is stable if it owns mass enough to connect all the included galaxies. These galaxies should have an inferior speed, in comparison to a precise stability limit. Scientists discovered that in many galactic clusters, the galaxies are able to reach speeds really superior to this limit of stability, but they still maintain a stable structure. This implies the presence of a really intense gravitational field, able to tie together these galaxies. We are not able to explain this field with the visible matter. The cluster should be tied together by an invisible matter, really more abundant than the bright one.
  • Gravitational lenses. The gravitational lenses are objects, or groups of objects, endowed of an extremely large mass, whose gravitational field is intense enough to deflect the course of the nearest beams. As a result, if one of these “lenses” would be placed between us and a far light source (for instance, a quasar), we will be able to admire more images for the same object. This kind of optical illusion allows us to discover the potential dark object which deviated the light.

Anyway, which is the composition of this dark matter? It may be normal matter, for instance, planets or “brown dwarfs”, so small that they are unable to trigger the nuclear fusion reactions on their inside. Nevertheless, these small objects are really too few to be the real cause of the effects observed in the dark matter.
A different hypothesis concerns mass-fitted neutrinos. Scientists believe, in fact, that neutrinos are particles without mass, but recent experiments seem to indicate that they hidden a mass, even if very small (1/5000 of an electron mass). Being very diffused in the Universe, neutrinos would be enough to justify all the observed effects of the dark matter. An other type of possible obscure matter is made of particles, whose existence has still not been proved, called WIMPS (Weakly Interacting Massive Particles).

 

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From the Multimedia section

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