Dark energy is the name given to the unknown component driving the universe’s accelerated expansion.
In 1998, two independent teams studying distant Type Ia supernovae found that the expansion of the universe was speeding up rather than slowing under gravity. The discovery earned the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess.
The simplest explanation treats dark energy as a cosmological constant: a constant energy density associated with empty space. Other possibilities include changing fields or modifications to gravity, but observations currently fit the cosmological-constant model well.
Dark energy is often confused with dark matter. Dark matter clumps around galaxies and exerts attractive gravity. Dark energy appears smooth on large scales and is associated with accelerated cosmic expansion. Together with ordinary matter and radiation, these components shape the universe’s history.