  Ordinary baryonic particles (such as protons and neutrons) account for only
one-sixth of the total matter in the Universe. The remainder is a mysterious
"dark matter" component, which does not interact via the electromagnetic force
and thus neither emits nor reflects light. However, evidence is mounting for
its gravitational influence. The past few years have seen particular progress
in observations of weak gravitational lensing, the slight deflection of light
from distant galaxies due to the curvature of space around foreground mass.
Recent surveys from the Hubble Space Telescope have provided direct proof for
dark matter, and the first measurements of its properties. We review recent
results, then prospects and challenges for future gravitational lensing
surveys.
