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.
The cosmological constant is the simplest explanation, but it is not the only one on the table. Some physicists propose quintessence, a dynamic field whose energy density can change over time rather than staying perfectly constant, while others suspect general relativity itself might need modification on the largest cosmic scales instead of invoking a new form of energy at all. Distinguishing between these possibilities requires extremely precise measurements of how the universe’s expansion rate has changed over billions of years, which is exactly what missions like the European Space Agency’s Euclid telescope and the NSF-DOE Vera Rubin Observatory are now working to pin down, mapping the positions of billions of galaxies to trace dark energy’s fingerprint across cosmic history.
The term ‘dark energy’ itself was coined in 1998 by cosmologist Michael Turner, deliberately echoing the earlier and equally mysterious ‘dark matter,’ even though the two turned out to be entirely unrelated phenomena. Turner has since said he chose the name partly because it was catchy and easy to remember, a decision that stuck permanently even as the underlying physics remains just as unresolved today as it was then.