The Signature of Dark Matter in Quasar Light Curves

The idea that dark matter in the form of small compact bodies (most plausibly planetary mass primordial black holes) betrays its presence by the microlensing of quasars, has been stimulated by statistical studies of quasar light curves and gravitationally lensed quasar systems. In this paper we review this evidence, and discuss the significance of the recent detection of a planetary mass microlensing event in the Galaxy by the MACHO group. We also look in more detail at the light curves themselves, and make the case that many show the characteristic morphology of caustic crossing events associated with a large optical depth of lenses. This supports the idea that a large fraction of the critical density is in the form of planetary mass compact bodies. The nature of such bodies is discussed, and it is argued that current work on the QCD phase transition suggests they are primordial black holes, although it is just possible that they are compact gas clouds.