We expected the attractive force of gravity to slow down the rate at which the universe is expanding. However, observations made at Keck Observatory of very distant exploding stars show that the expansion rate is actually speeding up.
Over the largest distances, the universe seems to be dominated by a mysterious, repulsive “dark energy” that stretches space itself faster and faster with time. But the physical origin and nature of dark energy, which makes up about 70 percent of the contents of the universe, may be the most important unsolved problem in all of physics.
The most recent data yield an additional surprise: the current rate of expansion is even faster than expected, perhaps showing that dark energy is growing stronger or revealing a new type of fundamental particle.
Professor of Astronomy
University of California, Berkeley