In cosmology, dark energy is a hypothetical form of
energy which permeates all of space and has strong
negative pressure. According to the theory of
relativity, the effect of such a negative pressure is
qualitatively similar to a force acting in opposition
to gravity at large scales. Invoking such an effect is
currently the most popular method for explaining
recent observations that the universe appears to be
expanding at an accelerating rate, as well as
accounting for a significant portion of the missing
mass in the universe.

Two proposed forms for dark energy are the
cosmological constant, a constant energy density
filling space homogeneously, and quintessence, a
dynamic field whose energy density can vary in time
and space.
Distinguishing between the alternatives requires high-
precision measurements of the expansion of the
universe to understand how the speed of the expansion
changes over time. The rate of expansion is

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