The only way these outer regions could remain intact was if they were bound together by the gravitational embrace of a much larger halo of invisible material, five to 10 times as massive as the visible galaxy. (discovermagazine.com)
Dark matter Invisible material thought to make up the majority of the mass of the universe. (discovermagazine.com)
HDST will make the first maps of the nearly invisible material permeating the cosmic web and fueling the growth of galaxies, thanks to its unprecedented collecting area, its 100 × gain in ultraviolet sensitivity over Hubble, and its novel multiplexed instrument modes. (hdstvision.org)