Washington Post: “Since 1990, the Hubble Space Telescope has offered scientists an incredible view of the universe. It has provided well over 1 million observations. Now, NASA has released “Hubble Focus: The Dark Universe,” a free e-book that explores what the Hubble mission has taught scientists about dark matter and dark energy — and how those lessons are shaking up long-standing theories. We can’t see it, but dark matter is thought to make up most of the mass of the universe. Unlike normal matter, which has mass, takes up space and is visible either by the human eye or through a telescope that shows other wavelengths, dark matter is invisible and mostly undetectable. For years, researchers have been teasing out its role in the universe, and astronomers now believe dark matter makes up 27 percent of the universe. Dark energy, which is thought to be responsible for the expansion of our universe, is thought to make up 68 percent.”