Abstract
This lecture describes an alternative to the dark matter model, the modified gravity model. There are a large number of models, but the one that best represents galaxies and solves the problem of the standard model at small scales is the MOND model proposed by Milgrom in 1983. The modification principle concerns weak fields, when the acceleration of gravity is less than a universal critical acceleration, of the order of 10-10 m/s2. This regime is not encountered either on Earth or in the solar system, but you have to go to the edges of the Galaxy to start encountering it, and only astrophysics can provide information on gravity in this regime. The model's successes are described for galaxies, dwarf, spiral or elliptical, whatever their type or mass. The development of bars and spirals is well reproduced, as are tidal interactions between galaxies and mergers. The model encounters problems for galaxy clusters, where mass is still missing, albeit less than in the Newtonian model. This mass could be a certain amount of neutrinos, or it could be black baryons, as over 80% of baryons are outside galaxies, and we don't yet know exactly where. Encounters between galaxy clusters allow the various materials to be separated, and could enable the various models to be tested further.
