The hypothesis of dark (or transparent) matter has been introduced by coherence with the mass estimates of galaxies, which are based on astronomical observations. Of a totally unknown nature, it is thought to be about five times more abundant than ordinary matter.

So what are the properties of dark matter? The observations clearly exclude the possibility of "hot" dark matter, i.e. with a speed very close to the speed of light at the time of the formation of the structures of the universe. However, the theory around cold dark matter is hindered by several discrepancies with observations, which has led to the development of alternatives including Warm Dark Matter (WDM) and Fuzzy Dark Matter (FDM). The WDM hypothesis is compatible with a hypothetical fourth family of neutrinos known as 'sterile', while FDM is compatible with string theory.

Physicists from IRFU have tested these two theories by studying the large structures of the universe. Their study included computer simulations of the formation of large structures of the universe based on various dark matter scenarios (cold, warm, fuzzy), performed at the CEA TGCC center for high-performance computing and Big Data.

To compare simulations and observations, the scientists chose the quasars—distant, highly brilliant sources which "illuminate" the intergalactic gas that separates them from Earth. They focused on hydrogen, an element they could study due to its absorption spectrum, called "Lyman-alpha forest". Using data collected by the Baryon Oscillation Spectroscopic Survey (BOSS) at the Sloan telescope in the US, and the Very Large Telescope (VLT) in Chile, they were able to reconstruct the large structures of the universe at a relevant scale.

Their results show that the best adjustment is obtained for cold dark matter. Through in-depth statistical analyzes of the many areas of uncertainty, they were able to obtain the best constraints to date on the properties of sterile neutrinos, which still remain hypothetical.