In 1998, astronomers discovered that the universe is expanding at an accelerating rate, thanks to a mysterious force called dark energy that makes up about 70% of the universe. The discovery came as a surprise as it countered the prevailing notion that the universe’s expansion should be slowing down because of gravity.
The breakthrough, which astronomers achieved by making calculations with observations of specific kinds of exploding stars, called type 1a supernovae, received the Nobel Prize in Physics in 2011.