A phenomenon in which irregularities in the distribution of mass in the early Universe give rise to localized temperature variations in the cosmic microwave background. It occurs because photons are gravitationally redshifted by regions where the density of matter is higher than average. The ripples detected by the Cosmic Background Explorer (COBE) satellite in 1992 are a manifestation of this effect. It is named after the American astrophysicists Rainer Kurt Sachs (1932– ) and Arthur Michael Wolfe (1939–2014), who published the idea in 1967. The integrated Sachs–Wolfe effect (ISW) refers to the same phenomenon as applied to photons from the cosmic microwave background on their journey through the Universe. If the Universe were dominated by matter, the ISW would be negligible, because the gravitational wells the photons fell into would on average be the same as the ones they climbed out of; hence their gravitational redshifts and blueshifts would cancel each other out. In a Universe dominated by radiation or dark matter the rate of expansion changes, thereby breaking this symmetry and resulting in a detectable ISW. The existence of the ISW adds support to the idea that the Universe is dominated by dark energy.