In addition, theories of cosmology predict the distribution of large scale structure , the distribution of matter in the universe. Weak lensing is a useful complement to measures of the distribution of luminous mass such as galaxy surveys . Lensing measures all the mass, in particular the dark matter as well as the luminous matter. This lensing can have effects in many measurements, as sources which would have otherwise been too dim become visible. This can be helpful, as when one wants to view objects that would otherwise be too far away.
Observations are underway by many groups.
See these notes by Penny Sackett on microlensing for more information. An introduction by Peter Newbury One can lens an astrophysicist , set up by Pete Kernan. NASA has a note on gravitational lensing. More technical reviews on the web include review talks from the Garching 1998 meeting, in particular the lecture by Bartelmann . See also the lectures by Bartelmann and Narayan.