Dark matter pulls, dark energy pushes — and both remain two of the greatest mysteries in modern physics. The realization that most of the universe is composed of a force that makes gravity repel rather than attract was a scientific breakthrough, according to Nobel Prize-winning astronomer Brian Schmidt.
Schmidt, along with fellow scientists Adam Riess and Saul Perlmutter, discovered this mysterious force—later named dark energy—in 1998. Their work fundamentally shifted our understanding of the cosmos and earned them the Nobel Prize in Physics in 2011.
Speaking during a visit to Ashoka University for the Lodha Genius Programme, Schmidt explained, “Dark energy represents energy embedded within space itself. Without it, the universe wouldn’t be accelerating, and its structure would appear completely different. It has a profound impact on how we perceive galaxies and cosmic expansion.”
The term dark energy was coined to echo dark matter, though the two are quite different. While dark matter refers to invisible particles that hold galaxies together through gravitational pull, dark energy does the opposite — it exerts a repulsive force, pushing the universe apart.
“Atoms and dark matter work together to pull the universe inward,” said Schmidt, “but dark energy is pushing outward. There’s an ongoing cosmic tug-of-war — and dark energy appears to have taken the lead.”
This dominance began around 6.5 billion years ago, Schmidt noted. Although dark energy’s density has remained constant since the Big Bang—13.8 billion years ago—the density of matter and atoms has gradually declined as the universe expanded. At a certain point, dark energy’s influence surpassed that of matter, leading to the current acceleration in cosmic expansion.
The research journey began in 1994, with Schmidt and his team attempting to chart how the universe’s expansion rate had changed over time. By observing distant celestial objects, they hoped to determine whether the universe was slowing down rapidly—suggesting a heavy universe doomed to collapse in a “Gnab Gib” (Big Bang in reverse)—or slowing gently, implying a lighter universe destined to expand forever.
But just three and a half years into the project, they found something entirely unexpected: the universe wasn’t slowing down at all—it was speeding up. The discovery of this acceleration pointed directly to the existence of dark energy, reshaping cosmological theories for generations to come.