~5 billion years agoReputable sourceWell documented
Dark Energy and the Accelerating Universe
On the timeline · around ~5 billion years ago · The Age of Galaxies
What happened
For most of cosmic history the pull of gravity slowed the universe's expansion. But in the 1990s, studies of distant exploding stars revealed that the expansion is instead speeding up. The cause was named dark energy — a mysterious, repulsive influence that makes up roughly 70% of the universe and, over the last several billion years, has come to drive its accelerating growth.
Why it matters
The discovery of cosmic acceleration, honored with the 2011 Nobel Prize in Physics, revealed that most of the universe is made of something no one yet understands — one of the deepest open questions in science.