Even weirder than dark matter—the invisible stuff constituting most of the mass of the universe—is dark energy, a mysterious force pushing the universe apart at an ever-faster rate. Weirder still is a recent discovery that dark energy has been around for most of the history of the cosmos. “Nine billion years ago, dark energy was already wielding its repulsive influence on the universe,” explains Johns Hopkins University astrophysicist Adam Riess. But the repulsion didn’t win out against the force of gravity until 5 billion years ago, when cosmic expansion kicked into high gear and began accelerating.

Calibrating images from the Hubble Space Telescope’s high-end cameras against those from the Wilkinson Microwave Anisotropy Probe—a satellite that maps the heat signature of the early universe—Riess and his colleagues retraced the growth history of the universe with unprecedented accuracy and depth. “It’s as if you mark the height of a child against a doorframe to measure growth spurts,” Riess says. “While dark matter retards expansion, dark energy propels it.”

For reasons as yet unknown, the anti-gravitational effects of dark energy are greater now than they were in the distant past. One theory, which the Hubble data support, is that empty space is pregnant with residual energy from the Big Bang. As space expands, there is more dark energy, while matter becomes more spread out, weakening the inward pull of gravity. But a universe of questions still remains. “This is another clue,” Riess says, “and we know so little about dark energy that anything we can find out is important.”