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A phenomenon predicted by Albert Einstein in his general theory of relativity that causes light to bend around massive celestial objects. For example, a near galaxy curves space-time around it to such an extent that light can be seen from a distant quasar even though it's behind the galaxy.

LRG 3-757 Einstein Ring caused by gravitational lensing (credit:- NASA, The Hubble Heritage Team (AURA/STScI)

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