Third milestone: 603 light years from Earth
The Ursa Minor Dwarf Galaxy is a satellite of the Milky Way, about 200,000 light years from Earth, that was discovered nearly sixty years ago. This galaxy is so small and faint that astronomers believe it contains mainly dark matter.
The nature of dark matter is a great mystery, but according to our current best guess it is a new kind of subatomic particle that neither emits nor absorbs electromagnetic radiation.
Astronomers are using NASA's Fermi Space Telescope to look for tiny bursts of high energy gamma rays, emitted by dark matter particles as they annihilate each other, in galaxies like the Ursa Minor dwarf.
Main image credit: NASA Extragalactic Database