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Weak lensing (weak gravitational lensing, WL) is the detection of astronomical 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), the patterns being due to distortion by gravitational lensing. Both the shape and the position of these background objects are distorted. It is termed weak in that while it distorts the view, it does not go so far as to create multiple images of the same object, which is called strong lensing (strong gravitational lensing).
Surveys use weak lensing to study the early universe and cosmology. Galaxy clusters are detected through patterns in the position and shape of objects apparently surrounding them, but also can be detected through weak lensing of the cosmic microwave background (CMB), CMB weak lensing, such lensing adding to the variations (anisotropies) already present the passing CMB radiation.
Evidence of weak lensing may be statistical: shapes and locations in the sky that suggest weak lensing's distortion may occur more frequently than would be expected without the occurrence of WL. Useful information may be gleaned through statistical analysis.