In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. Wikipedia

Repulsive property, theorized to make up most of our universe in order to explain its observed expansion; has constant energy density Wikidata

A hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. OmegaWiki

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. Wiktionary

Hypothetical form of energy which, it is supposed, is spread uniformly throughout space and time and has anti-gravitational properties. Wiktionary (translation)