Dark energy may compose roughly 68% of the universe, but scientists know even less about it than they do about dark matter. But something like dark energy must exist to explain the universe’s accelerating expansion.
Since the late 1920s, astronomers have known that the universe is expanding. In the 1990s, observations of distant star explosions, called supernovae, showed that the universe expanded more slowly in the past than it does now. The reason for this remains unclear, but the leading explanation is that the universe contains something that has a repulsive gravitational effect – it pushes the universe apart instead of pulling it back together. This phenomenon is called dark energy.