The universe's expansion is speeding up, not slowing down

For most of the 20th century, cosmologists assumed the universe’s expansion must be gradually slowing as gravity pulled all that matter back together. The only question was whether it would slow enough to halt one day.

Then in 1998, two independent teams measuring distant exploding stars called Type Ia supernovae found those explosions were fainter, and therefore farther away, than expected. The startling conclusion was that the expansion of the universe is not decelerating at all but actually speeding up. Something is pushing space apart faster over time, and that something was named dark energy.

According to NASA, the acceleration kicked in roughly nine billion years after the universe began, once dark energy came to dominate over the gravitational pull of matter. Dark energy now makes up most of the cosmos, yet its underlying nature remains one of physics’ deepest open questions.

The discovery so reshaped cosmology that it was awarded the 2011 Nobel Prize in Physics.