  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.
