Cosmologists have known that the universe is expanding in all directions since early in the 1920s. Later in the century, they used new instruments to examine the question again. They assumed that—due to gravity—the rate of expansion of the universe today would be slower than the rate in the past. Instead, measurements showed that the cosmic expansion has been speeding up. The expansion is attributed to —dark energy,— a kind of springiness of empty space itself. Today's astronomical measurements show that dark energy makes up about 70% of the total mass-energy in the universe. This deep mystery lies right at the heart of understanding gravity. See how observations that measure the history of cosmic expansion more precisely, along with a more detailed look at the cosmic microwave background, are providing new clues about the nature of dark energy.