In cosmology, “dark matter” refers to a hypothetical component of matter that, unlike known matter, would emit electromagnetic radiation and is currently detectable only indirectly through its gravitational effects. The hypothesis arises to justify experimental observations that, relative to the laws of gravity, dark matter would account for 90 percent of the mass of the universe.
It should not be confused with another 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 luminosity and added up all the masses to obtain the total mass of the cluster; he then obtained a second independent estimate of the total mass based on measuring the dispersion of the individual velocities of the galaxies: this second estimate of the dynamic mass was 400 times larger than the previous one.
It was not until the 1970s that scientists began to analyze this discrepancy by hypothesizing the existence of dark matter, the discovery of which would solve the problem of the lack of mass in clusters of galaxies and could allow new hypotheses about the origin, evolution, and fate of the universe.
Potential evidence for the existence of dark matter includes: 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 remains constant, and if invisible mass did not exist, they would have to leave the galaxies. In the second case, we have the observation of gravitational lensing in the presence of insufficient visible mass to explain the phenomenon.
Scientists have developed several theories about the nature of the missing mass, which should be located in the blackness around stars and should be divided into baryonic (analogous to the matter of stars and planets, but unable to emit radiation) and non-baryonic (hypothetical WIMP particles). To date, no theory has been conclusively verified.