In 1917, Einstein introduced a term into his equation of state for the entire universe, which would balance the effects of gravity and keep the universe static. [Otherwise the equation predicted that the universe must either expand or contract.] Then in 1929 Edwin Hubble discovered the universe was in fact expanding, and Einstein dismissed his term as “the biggest physics blunder I have ever made.” Einstein's term was like a repulsive force intrinsic to space itself... space “naturally” wants to expand.

Two paths led to the discovery and measurement of “Dark Energy.” The first was the discovery by Penzias and Wilson in 1965 that they could see the entire universe at the moment it first became transparent to visible light, at the age of about 380,000 years.

Increasingly sophisticated satellite observations have given a very clear and increasingly sharper image of the entire universe at this time, and particularly the density fluctuations from which all arrangements of matter in the universe gradually evolved.

The other strand is the establishment of a very accurate cosmic distance scale using a type of supernova which has a standard brightness. Observations of very distant supernovae (occurring when the universe was much smaller, up to half its present size) show that the universe is accelerating in its expansion, compared to the characteristic slowdown we would expect if gravity were the only long-range force in the universe. The usual description of this accelerated expansion is in terms of that same dynamical property of empty space first suggested by Einstein in 1917... now usually called Dark Energy.

Combining all observations and fitting to models gives us for the first time a reliable idea of the actual composition of the entire universe!

There doesn't seem to be any way to understand this property of the vacuum in terms of the Standard Model, which seems to predict an expansion of space that is greater by 120 powers of 10 than what is seen. This failure is unsurprising, since the effective force for dark energy has about the same strength as gravity, and gravity itself is nowhere present in the Standard Model.

COBE

WMAP

Planck