Until the supernova findings, many astronomers assumed that the long-term evolution of space would constitute one of two possibilities depending on the amount of mass within it: either continuing to expand forever at a slower and slower pace, slowed by the mutual gravitational attraction of all its matter and energy, or, if its density exceeds a certain critical amount, reversing course and recontracting down to a crunch. The options resembled a roller coaster nearing the end of its track. Virtually everyone expected a gradual slowdown, followed perhaps by a backward ride. Few thought the vehicle would be charging full speed ahead.

Ordinary gravity cannot account for such acceleration. As an attractive force, it acts to clump massive objects together, putting brakes on the outward motion of the galaxies. Because both dark and visible forms of matter interact on the basis of gravitation, they could not engender the repulsive forces required to push galaxies apart. Turner and other researchers rapidly reached the conclusion that a new type of substance must be at work, one that creates a kind of cosmological antigravity. They dubbed the unknown agent “dark energy” to distinguish it from dark matter.

There are several important differences between dark matter and dark energy. While dark matter is thought to have an uneven distribution, mainly clumped around visible population centers (with a lesser amount sprinkled throughout the void), dark energy is believed to be as smooth as custard, spread uniformly throughout space. Otherwise, in contrast to known observations, the universal expansion would exhibit distinct behavior in various directions.

Moreover, although the composition of dark matter is largely unknown, scientists have put forth an array of likely candidates. Any massive, but elusive, particle present in sufficient quantities could potentially fit the bill. A prime example is the neutrino, a fast-moving particle believed to comprise at least a portion of dark matter.

By comparison, dark energy contenders have been much harder to identify. Proposed explanations have called on entirely new



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