2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/219289To the astrophysicist faced with the puzzle of dark matter, this one appears under two different aspects: on the one hand in cosmology, i.e. at very large scales, where it seems to be made of a bath of particles; on the other hand at the scale of galaxies, where it is described by a set of specific phenomena, looking incompatible with a description in terms of particles, and suggesting that we are seeing a modification of the law of gravity. Reconciling these two distinct aspects of dark matter in a single theoretical formalism is an important challenge which could lead to some new physics in action at astronomical scales.6 pages, to appear in l'Astronomie (in French)Cosmology and Nongalactic AstrophysicsGeneral Relativity and Quantum Cosmologytext