In 1998 two teams of astronomers, measuring the distances to exploding stars billions of light-years away, expected to find that the expansion of the universe was slowing down, braked by the gravity of everything in it. Instead the supernovae were fainter than expected. They were further away than they should have been. The expansion was speeding up.
Saul Perlmutter, who led the Supernova Cosmology Project, and Brian Schmidt and Adam Riess, of the High-Z Supernova Search Team, shared the 2011 Nobel Prize in Physics for the discovery. Whatever drives the acceleration was soon named dark energy, a term coined by the cosmologist Michael Turner in 1998.
Two-thirds of everything
Dark energy is extremely thin, about 7 × 10⁻³⁰ grams per cubic centimetre, the equivalent of a few hydrogen atoms per cubic metre. But it fills all of space, and it does not dilute as the universe expands. Added up, it makes up about 68% of the energy content of the universe. Dark matter is about 27%, and all the ordinary matter of stars, planets and people is only about 5%.
Einstein’s constant
The simplest explanation is a cosmological constant, a term Albert Einstein added to his equations in 1917 to keep the universe static. When it was found that the universe is expanding, he dropped it. According to George Gamow, Einstein later called it his “biggest blunder”, though there is no direct record of him saying so. The constant is now back: a fixed energy belonging to empty space itself.
The trouble is the size. Quantum physics predicts that empty space should have energy, but the simple estimate comes out about 10¹²⁰ times larger than what is measured. It has been called the worst prediction in the history of physics, and nobody knows how to fix it.
Is it changing?
If dark energy is a true constant, its strength never changes. In 2024 and 2025 the Dark Energy Spectroscopic Instrument (DESI), which has mapped millions of galaxies, found hints that dark energy may be weakening over time. The evidence ranges from 2.8 to 4.2 standard deviations depending on the data combined, short of the five usually required for a discovery. If confirmed, it would be one of the biggest changes in cosmology in decades. The Vera C. Rubin Observatory and ESA’s Euclid telescope are now measuring the expansion more precisely to find out.