In early 1998, two independent teams of astronomers — the High-Z Supernova Search Team led by Brian Schmidt and Adam Riess, and the Supernova Cosmology Project led by Saul Perlmutter — measured the brightness of Type Ia supernovae exploding in distant galaxies. Type Ia supernovae act as "standard candles" because they reach nearly the same peak luminosity, so their measured brightness is a direct gauge of distance. The expectation was simple and intuitive: the gravity of all the matter in the universe should be slowing cosmic expansion.

The data said the opposite. The distant supernovae appeared markedly fainter than a decelerating universe would predict — meaning they were farther away than expected. The only consistent explanation was that the expansion of the universe is not slowing but has in fact been accelerating for about the past five billion years. The component driving this acceleration, whose physical nature remains unknown, was named "dark energy." Later observations of the cosmic microwave background and large-scale structure have continued to confirm the result; in today's standard Lambda-CDM model, the universe is roughly 68% dark energy, 27% dark matter and only 5% ordinary matter.

The discovery was recognized with the 2011 Nobel Prize in Physics, shared by Perlmutter, Schmidt and Riess. The Nobel committee stressed that the result emerging independently from two competing teams was decisive; it was the agreement between two separate measurements, not a single observation, that secured the finding.

Science sometimes advances not by answering a question but by opening a larger one. Dark energy is now considered one of the deepest open problems in twenty-first-century physics: the value predicted for the vacuum energy by quantum field theory differs from the observed value by roughly 120 orders of magnitude. The next generation of projects — the Euclid space telescope, the Vera Rubin Observatory and the Roman Space Telescope — has been designed specifically to narrow that gap.