Fifteen year ago, two teams of astrophysicists announced that distant objects were farther away than theoretical models said they should be, and so the universe must be expanding at a faster rate. Before that research, scientists thought all the matter in the universe would eventually stop cosmic expansion and lead to a contracting universe — after all, the force of gravity attracts objects to one another. But the fact that objects were getting farther apart instead of closer together meant that something is speeding up the expansion. And so, because scientists don’t know much about this something except that they can’t see it and it has an energy that opposes gravity, they named it “dark energy.”
Learn more about this mysterious force in “What is dark energy?” by Steve Nadis.