How did observations of Type Ia supernovae provide evidence for dark energy?

Observations of Type Ia supernovae provided the first direct evidence for dark energy by revealing that the expansion of the universe is accelerating.51

Because Type Ia supernovae possess a remarkably uniform intrinsic brightness, they serve as "standard candles" that allow astronomers to measure distances to galaxies across vast scales. In 1998, two independent research teams—the Supernova Cosmology Project, led by Saul Perlmutter, and the High-Z Supernova Search Team, led by Brian Schmidt and Adam Riess—observed that distant supernovae were 20–25 percent fainter than expected.1234

This dimness indicated that these supernovae were significantly farther away than they would be in a universe where the expansion rate was constant or slowing down due to gravity. The data implied that the expansion of the universe is speeding up, a phenomenon attributed to a gravitationally repulsive force known as dark energy, which acts as the dominant component of the universe's mass-energy budget.52413