A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing.
Amount of bending is one of the predictions of Albert Einstein's general theory of relativity.
The phenomenon occurs when a huge amount of matter, such as a massive galaxy or cluster of galaxies, creates a gravitational field that distorts and magnifies the light from objects behind it, but in the same line of sight.
These large celestial objects will magnify the light from distant galaxies that are at or near the peak of star formation.
Gravitational lensing is useful to cosmologists because it is directly sensitive to the amount and distribution of dark matter.
Lensing can therefore help astronomers work out exactly how much dark matter there is in the Universe as a whole, and also how it is distributed.