As the light emitted by distant galaxies passes by massive objects in the universe, the gravitational pull from these objects can distort or bend the light. This is called gravitational lensing.
Strong gravitational lensing can actually result in such strongly bent light that multiple images of the light-emitting galaxy are formed.
Weak gravitational lensing results in galaxies appearing distorted, stretched or magnified. Although difficult to measure for an individual galaxy, galaxies clustered close together will exhibit similar lensing patterns.
Analysing the nature of gravitational lensing patterns tells astronomers about the way dark matter is distributed within galaxies and their distance from Earth. This method provides a probe for investigating both the development of structure in the universe and the expansion of the universe.