What is dark energy?
Dark energy is the name for an unknown force that is making the expansion of the universe speed up. It makes up about 68 percent of the total energy of the universe.
In 1998, two teams of astronomers studying distant exploding stars called Type Ia supernovae discovered that the universe's expansion is accelerating, not slowing down as expected. This surprising finding earned Saul Perlmutter, Brian Schmidt and Adam Riess the Nobel Prize in Physics in 2011.
Something must be pushing galaxies apart faster over time, and scientists call it dark energy. One explanation is that it is a property of empty space itself, called the cosmological constant, an idea first suggested by Einstein. Other theories suggest it is a changing energy field.
Is it changing?
In 2024 and 2025, results from the Dark Energy Spectroscopic Instrument, which mapped millions of galaxies, hinted that dark energy may be weakening over time. If confirmed, this would change our understanding of the universe's future. Ordinary matter makes up only about 5 percent of the universe.
Did You Know?
Depending on how dark energy behaves, the far future could see galaxies drift so far apart that no other galaxies are visible from the Milky Way.