Gravitational Lensing
Einstein’s theory of general relativity predicts that space is actually warped around massive objects.
In 1979, astronomers noticed two remarkably similar quasars very close together. They had the same magnitude, spectra, and red shift. They wondered if the two images could actually represent the same object. It turned out that a galaxy lay directly in the path between the two quasars and Earth, much closer to Earth than the quasars. The geometry and estimated mass of the galaxy were such that it produced a gravitational lens effect that is, a warping of the light as it passes through the space around the galaxy.

Many other instances of gravitational lensing have now been detected. Gravitational lensing can produce more than two images, or even arcs. Images produced by point-like gravitational lenses can appear much brighter than the original source would appear in the absence of the gravitational lens.