What does "dark matter" mean?
Although the term "dark matter" might sound mysterious, it is simply the term used by physicists, astrophysicists and cosmologists to describe material which does not emit visible light, radio waves, X-rays, gamma rays or other electromagnetic radiation. Nowadays, we believe that there is a lot more dark matter than visible matter and that it is in part responsible for the structure of the Universe that we can see.
The first evidence for the presence in the Universe of matter which we cannot see goes back to studies carried out in the 1930s into clusters of galaxies, i.e. groups of many galaxies separated by huge empty spaces. Zwicky, in 1933, and Smith, in 1936, while studying great groups of galaxies (respectively the galaxy of the Chioma di Berenice and the galaxy of Virgo), observed that the speeds of the galaxies were much higher than those expected for a gravitationally linked system, whose mass was only that of the visible galaxies. They concluded that the galaxies in the cluster must be held together by gravitational effects due to a large amount of invisible matter, which they called "missing matter". This term is no longer used today; we prefer to talk about dark matter, i.e. which cannot be seen by emitted light.
It was not until the 1970s, however, with more reliable experimental data, a greater number of galaxies being sampled, and especially thanks to detailed studies into the motion of stars in the galaxies, that the scientific community was forced to take note of the question of dark matter.
It must be emphasised that all the astrophysical analyses mentioned assume the validity of the universal law of gravity even at very large distances. If the dependence of the gravitational force on the distance R were different from 1/R2, many of the conclusions reached regarding dark matter would have to be revised.