On the existence and the density of intergalactic dust

A relation between the redshift z of QSO's and their colour index ( B–V )' corrected for line emission and galactic absorption is interpreted in terms of an intergalactic selective extinction. The observed amount of extinction corresponds to a density of intergalactic dust grains of about 10 −34 to 10 −33 g cm −3 at z = 0. The life‐time of these particles are estimated to be longer than the Hubble ‐time at present. But at large z the life‐time of the grains considerably depends on the flux density of cosmic rays. Some implications of the existence of intergalactic dust are discussed.