In 1998, two teams of astronomers were measuring the expansion of the universe. They expected it to be slowing down. Instead, they found it was speeding up. Something was pushing galaxies apart. That something is called dark energy.

Dark energy makes up about 68 percent of the universe. It is not matter. It does not clump or cluster. It appears to be a property of space itself, a repulsive force that works against gravity. The simplest explanation is the cosmological constant, a term Einstein added to his equations and later called his greatest blunder. Now it looks like he was right.

The evidence comes from supernovae, the cosmic microwave background, and the large-scale distribution of galaxies. All three point to the same conclusion: the universe is accelerating. The discovery won the Nobel Prize in Physics in 2011.

What dark energy actually is remains unknown. It might be a constant. It might vary over time. It might be a sign that general relativity needs modification. The answers will shape our understanding of the universe's fate.