  The aim of this paper is to provide a plausible explanation for the large amplitude microlensing events observed in the cluster lensed quasar system SDSS J1004+4112. The microlensed quasar images appear to lie well clear of the stellar population of the cluster, raising the possibility that the cluster dark matter is composed of compact bodies which are responsible for the observed microlensing. In the first part of the paper we establish the exact structure of the difference light curves attributed to microlensing from photometric monitoring programmes in the literature. We then show from measures of surface brightness that the probability of microlensing by stars in the cluster is negligibly small. Finally we relax our assumption that the cluster dark matter is in the form of smoothly distributed particles, but instead is made up of compact bodies. We then use computer simulations of the resulting magnification pattern to estimate the probability of microlensing. Our results show that for a range of values for source size and lens mass the observed large microlensing amplitude is consistent with the statistics from the simulations. We conclude that providing the assumption of smoothly distributed dark matter is relaxed, the observed large amplitude microlensing can be accounted for by allowing the cluster dark matter to be in the form of solar mass compact bodies. We further conclude that the most plausible identity for these bodies is primordial black holes. 
SDSS J1004+4112: the case for a galaxy cluster dominated by primordial   black holes