“Don’t you understand?” Roberts said. “The galaxy has ended, but the velocities are flat.” He gestured at the points he’d plotted. “What is the mass out there? What is the matter? There’s got to be matter there.” They all stared at the photograph. Here was this beautiful swirl of billions of stars—the kind of majestic image that had captivated astronomers for more than half a century—though that’s not where they were looking. They were looking beyond it. Beyond the bulge, beyond the stars, beyond the gas of the spiral arms—beyond all of the light, whether optical or radio.
