Weak lensing is a phenomenon in astrophysics where the gravitational field of massive objects, such as galaxies or galaxy clusters, distorts the light from more distant objects as it passes by them. This distortion, known as gravitational lensing, can be used to study the distribution of dark matter in the universe, as well as to measure the masses of individual galaxies and galaxy clusters. Weak lensing is called "weak" because the distortions it causes are subtle and require careful statistical analysis to detect. Researchers use weak lensing to better understand the large-scale structure of the universe and to test theories of cosmology and gravity.