In physical cosmology, astronomy and celestial mechanics, dark energy
is a hypothetical form of energy that permeates all of space and tends
to increase the rate of expansion of the universe. Dark energy is the
most accepted theory to explain recent observations and experiments
that the universe appears to be expanding at an accelerating rate. In
the standard model of cosmology, dark energy currently accounts for
73% of the total mass-energy of the universe.

Two proposed forms for dark energy are the cosmological constant, a
constant energy density filling space homogeneously, and scalar fields
such as quintessence or moduli, dynamic quantities whose energy
density can vary in time and space. Contributions from scalar fields
that are constant in space are usually also included in the
cosmological constant. The cosmological constant is physically
equivalent to vacuum energy. Scalar fields which do change in space
can be difficult to distinguish from a cosmological constant because
the change may be extremely slow.

High-precision measurements of the expansion of the universe are
required to understand how the expansion rate changes over time. In
general relativity, the evolution of the expansion rate is
parameterized by the cosmological equation of state (the relationship
between temperature, pressure, and combined matter, energy, and vacuum
energy density for any region of space). Measuring the equation of
state of dark energy is one of the biggest efforts in observational
cosmology today.

Adding the cosmological constant to cosmology's standard FLRW metric
leads to the Lambda-CDM model, which has been referred to as the
"standard model" of cosmology because of its precise agreement with
observations. Dark energy has been used as a crucial ingredient in a
recent attempt to formulate a cyclic model for the universe.
