The universe is astonishing in multiple ways, and one of its most probative features is the existence of its dark side: dark matter, interacting only via gravity, dark energy, the mysterious so-called energy of empty space that is accelerating the expansion of the universe, and black holes, from which not even light can escape.

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Joseph Silk is Homewood Professor of Physics and Astronomy at Johns Hopkins University. He studied at Cambridge, earned his PhD from Harvard in 1968, was a postdoctoral fellow at Cambridge and Princeton, and taught at the University of California, Berkeley, and the University of Oxford.

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