Dark energy is the name given to whatever is causing the expansion of the universe to speed up rather than slow down over time. Astronomers expected gravity to gradually pull galaxies back together and decelerate the expansion, but observations of distant supernovae in 1998 showed the opposite was happening.
Whatever dark energy is, it appears to make up roughly seventy percent of the total energy content of the universe, dwarfing both ordinary matter and dark matter combined. The leading, simplest explanation treats it as a property of empty space itself, a constant energy density that does not dilute as the universe expands, called the cosmological constant.
Dark energy remains one of the biggest open mysteries in physics. Its existence was essentially unknown before 1998, and there is still no agreed upon explanation for what it fundamentally is, only strong observational evidence that it is there and that it is winning.