dark energy

Short definition from Wiktionary

Noun

1. 1
   A hypothetical form of energy thought to be spread uniformly throughout space (and time) and to have 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.