  We review progress in understanding dark matter by astrophysics, and
particularly via the effect of gravitational lensing. Evidence from many
different directions now all imply that five sixths of the material content of
the universe is in this mysterious form, separate from and beyond the ordinary
"baryonic" particles in the standard model of particle physics. Dark matter
appears not to interact via the electromagnetic force, and therefore neither
emits nor reflects light. However, it definitely does interact via gravity, and
has played the most important role in shaping the Universe on large scales. The
most successful technique with which to investigate it has so far been the
effect of gravitational lensing. The curvature of space-time near any
gravitating mass (including dark matter) deflects passing rays of light -
observably shifting, distorting and magnifying the images of background
galaxies. Measurements of such effects currently provide constraints on the
mean density of dark matter, and its density relative to baryonic matter; the
size and mass of individual dark matter particles; and its cross section under
various fundamental forces.
