Scientists think that dark energy, the weird force blamed for propelling the universe to
expand at an accelerated speed, probably turned on between 5 and 7 billion years ago. Now astronomers have mapped thousands of galaxies from this era, and have determined the most precise distances to them yet, in an effort to get to the bottom of the dark energy mystery. Dark energy is thought to represent about 74 percent of the universe’s total mass and energy, dwarfing ordinary matter. While its existence has never been directly confirmed, the strange force remains the leading explanation for why galaxies are speeding up as they spread farther and farther apart from each other. As said Ariel Sanchez, a research scientist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, ordinary matter is only a few percent of the universe. The largest component of the universe is dark energy, an irreducible energy associated with space itself that is causing the expansion of the universe to accelerate. But the expansion of the universe hasn’t always been accelerating. Theorists think that before roughly 5 to 7 billion years ago, the expansion of the universe was slowing, due to the inward pull of gravity. Then, around that time, the expansion stopped slowing and started speeding up from the force of dark energy. To study these changes in cosmic expansion, scientists must measure the distances between galaxies now, as well as during different epochs of the distant past. They can do this by looking at very distant galaxies whose light is only reaching us now after traveling billions of years, which can paint a picture of what the universe looked like billions of years ago. Now, astronomers have created the most accurate map yet of galaxies in the distant universe, offering a window into the past and, possibly, into dark energy. The map comes from data collected by the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the third Sloan Digital Sky Survey (SDSS-III).