Dark matter, in cosmology, indicates a hypothetical component of matter that, unlike known matter, would emit electromagnetic radiation and would currently be detectable only indirectly through its gravitational effects. The hypothesis was born to justify experimental observations according to which, in relation to the laws of gravitation, dark matter would constitute 90% of the mass of the Universe. Not to be confused with the different hypothesis called dark energy.
In 1933 the astronomer Fritz Zwicky, observing clusters of distant galaxies, estimated the mass of each galaxy in the cluster based on its brightness and added all the masses to obtain the total mass of the cluster, then obtained a second independent estimate of the total mass, based on the measurement of the dispersion of individual velocities of galaxies: this second estimate of dynamic mass was 400 times larger than the previous one.
Only since 1970 researchers began to analyze this discrepancy, hypothesizing the existence of dark matter, whose discovery would solve the problem of lack of mass in galaxy clusters and could allow new hypotheses on the origin, evolution and fate of the Universe.
The existence of dark matter
Among the potential evidence for the existence of dark matter are: the rotation of galaxies and gravitational lensing. In the first case it is observed that the orbital velocity of stars in the peripheral regions of many galaxies does not decrease with distance as it should (Kepler’s Second Law) but it remains constant and, if there was no invisible mass, they should leave the galaxies. In the second case we have the observation of gravitational lensing effects in the presence of a visible mass insufficient to justify the phenomenon.
Researchers have developed several theories about the nature of the missing mass that should be located in the black around the stars and distinguished in baryonic (similar to the matter of stars and planets, but not able to emit radiation) and non-baryonic (hypothetical WIMP particles). To date no theory can be verified in a conclusive way.
There are several clues that lead to speculate the existence of dark matter:
- Rotation curves of spiral galaxies. If all matter in spiral galaxies, systems in rapid rotational motion around their galactic center, were the bright matter (stars, gas clouds, interstellar dust), the areas very distant from the center (distances of the order of 100,000 light-years) should orbit very slowly, similar to the outermost planets of the solar system. Since instead we generally observe a persistence of the rotation speed up to the extreme observable periphery of the spirals, we deduce the existence of an amount of non-visible matter estimated 10 times higher than the visible matter, distributed perhaps in the form of a spheroidal halo up to distances much greater than those to which stars and gas clouds extend. Less evidence for the presence of dark matter exists for elliptical galaxies and other star systems.
- Gravitational lensing. Another possible proof of the existence of dark matter is given by the observation of gravitational lensing effects in the presence of a visible mass that is insufficient to justify them. In 2008 a group of researchers, including French and Canadians coordinated by the Institute of Astrophysics in Paris, using the telescope Canada-France-Hawaii Telescope (Cfht) located on Mount Mauna Kea in Hawaii, studied thousands of images noting the deviation that the light underwent even in points where there were no visible masses.
- Clusters of galaxies. It is possible to provide an approximate estimate of the total mass, visible and invisible, of a cluster of galaxies with purely dynamical methods, knowing only the velocities of the individual galaxies constituting the cluster. These estimates indicate the presence of dark matter in amounts 10 to 30 times greater than the matter emitting detectable electromagnetic radiation. If this dark matter were not present, the cluster would no longer be gravitationally bound and the constituent galaxies would rapidly dissipate.
- Peculiar velocities. Each galaxy is subject to gravitational attraction from all other surrounding galaxies, and its average velocity increases as the total density of the attractive mass increases. Even our galaxy, for example, ‘falls’ with velocities of several hundred kilometers per second toward the Virgo cluster, about 60 million light-years away. Observational estimates of the velocity field of our galaxy and neighboring galaxies lead to the assumption of dark matter in much larger amounts (10 or 100 times) than detectable matter, distributed on very large scales.
- Primordial nucleosynthesis. A fundamental prediction of cosmological theories is the production, in certain quantities, of the nuclei of the chemical elements, occurred during the primordial epoch in which the average temperature of the Universe was about 1010 K: the agreement with the relative abundances of some isotopes of helium, lithium and other elements observed spectroscopically in stars requires a density of total baryonic matter about 20 times higher than that observed.