In 1986, Prince ton’s Bohdan Paczynski suggested that if these massive objects we couldn’t see did exist in the halo of our own galaxy—where astronomers thought most of the Milky Way’s dark matter resided—we could recognize their presence through a technique called gravitational lensing. In 1936, Einstein had suggested that a foreground star could serve as a lens of sorts on a background star. The gravitational mass of the foreground star would bend space, and with it the trajectory of the light from the background star, so that even though the background star was “behind” the foreground star
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