Weak Lensing (Weak Gravitational Lensing) is the detection of objects by noticing patterns in the apparent positions of background objects that reveal an intervening object with mass, such as a Galaxy or Galaxy Cluster due to Gravitational Lensing. It is termed weak in that while it distorts the view, but does not create multiple images of the same object, which is called Strong Lensing (Strong Gravitational Lensing) to distinguish them.
Surveys using weak lensing are used to study the early universe and Cosmology. In particular, galaxy clusters are detected through weak lensing of the Cosmic Microwave Background, effectively adding to the variations (Anisotropies) already in the radiation, such as those dating from Recombination, called CMB Halo Weak Lensing.
Dark Energy Survey (DES)