these observers can calculate how much mass must have caused the curving. Offering an extraordinary tool for mapping out the hidden material in the cosmos, gravitational lensing has furnished ample evidence that luminous bodies comprise just a small subset of all that there is.

What is this mysterious substance that signs its name only with gravity’s mark? Early on, scientists supposed that it was nonshining stars, meaning those that either burned out or never had enough material to ignite in the first place. Examples of these would be objects called neutron stars (the ultracompact remnants of massive stellar cores) and brown dwarfs (stars comparable in size to very large planets, lacking the critical mass of hydrogen required to stoke the furnace of stellar fusion). Further measurements, though, have indicated that nonshining stars represent only a portion of the missing material. Most of the hidden stuff must be composed of new kinds of substances—rather than the ordinary matter, made of protons and neutrons, that constitutes stars and planets.

Cosmologist Michael Turner of the University of Chicago has offered a number of suggestions for what dark matter could be. At scientific conferences he lays out his ideas on colorful transparencies, wagering like a sports commentator which are the best bets. His prime candidate is a hypothetical particle called the axion, to which he gives high odds despite the fact that powerful detectors have searched for it in vain.

To further complicate matters, in 1998 an extraordinary astronomical discovery seemed to cast even more of the cosmos into shadow. Using precise measurements of the distances and velocities of supernovas (stellar explosions) in extremely remote galaxies, several teams of astronomers determined how the Hubble expansion changes over time. To their amazement, they found that the universe’s growth is speeding up as it ages. Not only is the cosmos ballooning outward, it is doing so faster and faster—with no end in sight.

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