In 1998 astronomers found the universe's expansion is speeding up, not slowing.
Everyone expected gravity to be gradually slowing the expansion of the universe. Two rival teams set out to measure how much, using exploding stars as distance markers. Both found the opposite: distant galaxies are receding faster over time. Whatever is driving it has been named dark energy, and nobody knows what it is.
Since the discovery of the cosmic microwave background in the 1960s, the Big Bang has been the leading account of how the universe evolved, with galaxies moving apart as space expands. The natural assumption was that the mutual gravity of all the matter in the universe would steadily brake that expansion. Cosmologists were so confident that they even defined a 'deceleration parameter' to measure it.
In 1998 two independent groups, the Supernova Cosmology Project and the High-Z Supernova Search Team, tested this with type Ia supernovae. These explosions of white dwarf stars reach almost the same peak brightness, so how bright one appears reveals its distance. Comparing that distance with how much the supernova's light had been stretched by cosmic expansion let them reconstruct the expansion history. The distant supernovae were fainter than expected, on average about 10 to 15 per cent farther away than a decelerating universe allowed. The expansion is accelerating. Three members of the teams later received Nobel Prizes.
Other evidence, including patterns in how galaxies cluster, has since backed the result. Within general relativity, the simplest explanation is a cosmological constant, a small energy built into empty space itself, known as dark energy. It is part of the standard model of cosmology, called Lambda-CDM, though alternative explanations are still being explored. The acceleration is thought to have begun roughly 5 billion years ago, when dark energy came to dominate over matter.
It also rewrites the universe's future. If dark energy stays constant, expansion continues forever and speeds up exponentially, and even the cosmic microwave background eventually fades beyond detection, leading to a cold 'heat death'. If dark energy grows stronger, some models predict a 'Big Rip' that tears everything apart. Which fate awaits depends on what dark energy turns out to be.
Source: Wikipedia — Accelerating expansion of the universe · Text summarised from Wikipedia (CC BY-SA 4.0)
