Supernovae Ia have been used to measure distances in the Universe. The evidence for an acceleration of the cosmological expansion was a surprise to many in 1998. This acceleration can be explained by a cosmological constant, however particle physicists interpretation of the cosmological constant as vacuum energy cannot explain the observed value. This has lead to postulate the existence of a Dark energy component.

The initial supernovae measurements have been confirmed by a number of observations with cosmological microwave radiation, gravitational lensing and correlations between large scale structures (from galaxies to clusters), leading to a cosmos where only 4 to 6% of the density of energy is understood.