Dwarf galaxy Leo IV, Hubble Space Telescope (HST) optical image. This small, dim galaxy in visible light is a sparse scattering of stars that are virtually indistinguishable from the background. This is one of more than a dozen ultra-faint dwarf galaxies found around our Milky Way galaxy. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the Universe's mass. The Leo IV dwarf galaxy is 500,000 light years from Earth, in the constellation of Leo. Image data obtained with the HST's Advanced Camera for Surveys (ACS) instrument. Image published in 2012.