  This paper argues that there is a large population of dark galaxies which
reveals its presence by the gravitational lensing of quasars, and outnumbers
normal galaxies by around 3:1. There are 8 double quasars with a separation
greater than 2 arcseconds which have been classified as probable or certain
gravitational lenses. Lensing galaxies have only been found for 2 of these
systems, and analysis of the remainder has led in each case to the conclusion
that they are best explained by the gravitational lensing effect of a `dark
galaxy'. Here we examine the ensemble properties of this sample and conclude
that there is overwhelming evidence that the systems are indeed being
gravitationally lensed by dark galaxies or perhaps dark matter galactic halos.
The existence and nature of such objects raises some intriguing questions, as
well as having profound implications for large scale structure. Several of the
quasar systems show strong evidence for microlensing. It is argued that this
implies a substantial component of dark matter in the form of compact bodies,
either in the halo of the lensing galaxy or more generally along the line of
sight.
