Dark matter could sling lethal meteors at Earth, potentially causing mass extinctions like the cataclysm that ended the Age of Dinosaurs, Harvard scientists say. Physicists think the mysterious, invisible substance called dark matter makes up five-sixths of all matter in the universe. It was first detected by the strength of its gravitational pull, which apparently helps keep the Milky Way and other galaxies from spinning apart, given the speeds at which they whirl. Scientists have recently suggested that a thin, dense disk of dark matter about 35 light-years thick lies along the central plane of the Milky Way, cutting through the galaxy's disk of stars. The sun travels in an up-and-down, wavy motion through this plane while orbiting the center of the galaxy. via Space.com.