Gravitational Lensing is one the important fields of Cosmology. Using gravitational Lensing is one the ways to explore Dark Matter of the universe. Here you can find a good review article about the week lensing by Bartelmann and Schneider
One of the active area of gravitational lensing is so called, Microlensing (For Review see Roulet, E. and Mollerach, S. (1997), Phys. Reports, 279, 67).
Studying the rotational curve of galactic disk provides the existence of dark matter in the halo of our galaxy. The Massive
compact halo astrophysical objects (MACHOs) like brown dwarfs,
white dwarfs, Neutron stars and black holes could be as baryonic candidates for the dark halo. The gravitational microlensing
technique for the first time proposed by (Paczynski 1986, APJ 304, 1.) to observe
indirectly MACHOs in the halo of our galaxy. Passing MACHOs through our line of
sight cause temporary magnification of background stars. Comparing the number of
observation by the expected number of microlensing events can obtain the
fraction of halo that made by MACHOs (astro-ph/0203037).
Here is the some experimental groups in this field:
EROS Project, MACHO Project, MOA Project, OGLE Project, PLANET Project, Microlensing Planet Search Project, Microlensing Pastimes