Dark energy is the hypothesized cause of the accelerating expansion of the universe, discovered by Saul Perlmutter, Brian Schmidt, and Adam Riess in 1998. They were awarded with the 2011 Nobel Prize in Physics for this discovery. Unlike dark matter and ordinary matter that exert gravity, dark energy must exert an anti-gravity effect that tends to push galaxies apart. According to the Planck data, the total substance of the present-day universe, is composed of about 68% dark energy, 27% dark matter, and 5% ordinary matter. Although it contributes the largest fraction of mass-energy in the universe, dark energy is extremely dilute. It has negligible effect on the solar system and at galactic scale, but its effect accumulates over the vast empty space between galaxies to provide the observed cosmic acceleration.
The cosmological constant, introduced by Einstein in 1918, has been the leading candidate for dark energy and is consistent with the recent observations by the Planck satellite mission. However, we do not know what could be a cosmological constant. The quantum vacuum energy is a natural candidate, except that it provides an energy density more than 120 orders of magnitude too large for the observed value for dark energy. Various theoretical attempts have been proposed to negate this embarrassment. So far no consensus has been reached.
Another dark energy candidate is a dynamical field, often generally referred to as quintessence. Being dynamical, quintessence can evolve in time and vary in space, albeit minutely; otherwise it would not be able to explain the observations. Further precision measurements are required to distinguish quintessence from cosmological constant and to determine the true nature of dark energy.