Intergalactic matter in galaxy clusters

Current knowledge is summed up of the form and origin of so-called hidden matter in galaxy clusters, whose existence stems from the application of the virial theorem to probably stable galaxy clusters. A value of 5x10-27 kg.m-3 was determined for the density of intergalactic dust from observational data, a value of 3x103 m-3 was determined for the density of hot gas particles. No positive results have so far been found in the search of a cooler gas. The mass of the intergalactic matter observed has been significantly lower than the required mass of hidden matter. The origin of the intergalactic gas and dust has not yet been explained. (Ha)