A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's general theory of relativity. If light is treated as corpuscles travelling at the speed of light, Newtonian physics also predicts the bending of light, but only half of that predicted by general relativity.
One of Eddington's photographs of the 1919 solar eclipse experiment, presented in his 1920 paper announcing its success
Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.
In the formation known as Einstein's Cross, four images of the same distant quasar appear around a foreground galaxy due to strong gravitational lensing.
This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J1206.
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-largest known gravitationally bound structures in the universe after some superclusters (of which only one, the Shapley Supercluster, is known to be bound). They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with galactic clusters (also known as open clusters), which are star clusters within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
Composite image of five galaxies clustered together just 600 million years after the Universe's birth
Galaxy cluster IDCS J1426 is located 10 billion light-years from Earth and has the mass of almost 500 trillion suns (multi-wavelength image: X-rays in blue, visible light in green, and infrared light in red).
Abell 2744 galaxy cluster – extremely distant galaxies revealed by gravitational lensing (16 October 2014).
Galaxy cluster SPT-CL J0615-5746.