Abstract
This introductory lesson provides an overview of the discovery of dark matter. In 1937, Swiss astronomer Fritz Zwicky noticed that galaxies in clusters such as Coma are animated by disordered velocities of an amplitude far greater than that predicted from the visible mass of the galaxies. There must then be 100 times more invisible mass in the cluster, or Newtonian gravity is no longer valid at these scales. Zwicky's ideas took the astronomical community 40 years to accept. The rotation of galaxies, measured with increasing precision in the years 1970, also reveals missing mass, such as black halos around individual galaxies. However, X-ray emissions from clusters reveal hot, diffuse gas whose mass can be up to 10 times that of the galaxies themselves. And the rotation of certain galaxies can be explained without missing matter. Is dark matter really well established ? The discovery of large reservoirs of atomic gas around galaxies, which, thanks to the hyperfine line at 21 cm, makes it possible to measure rotation speeds up to 5 to 10 times the optical radius, when there are no more stars, shows without a doubt the existence of dark matter. Around 1984, dark matter became exotic, and we realized that it could not be made up of ordinary particles. Both primordial nucleosynthesis, and everything we learn about primordial fluctuations just after the Big Bang, establish the new paradigm of cold dark matter, which is still in force today, and which will be the subject of lectures in 2015.
