However, in the present dark-energy era, it dominates the mass–energy of the universe because it is uniform across space.[94][95] Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[96] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space.


An explanation for why the expansion of the Universe is accelerating remains elusive. It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space.[93] On a mass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) is much less than the density of ordinary matter or dark matter within galaxies.

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