Gravitational lens

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general theory of relativity.
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Any massive celestial object, such as a galaxy or a cluster of galaxies, which distorts the spacetime continuum so that the light from more distant objects is bent as predicted in Einstein's General Theory of Relativity. This bending results in more than one image of the background object being visible to the foreground observer. The amount of lensing allows for an estimate of the mass of the lens to be made, which leads to the need for dark matter to exist, because more matter is there than can actually be seen. A symmetric gravitational imaging of a radio source is an Einstein ring.