dark energy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hypothetical form of energy that permeates all space and tends to cause the universe to expand by exerting a gravitational repulsion, countering ordinary gravitational attraction. Two forms of dark energy are quintessence and the vacuum energy represented by the cosmological constant.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hypothetical form of energy which, it is supposed, is spread uniformly throughout space (and time) and has anti-gravitational properties: it represents a possible mechanism for the cosmological constant, and thus is one of the possible explanations for the current accelerating rate of expansion of the universe; and it is estimated to account for about 74% of the mass-energy of the universe.