The Dark Galaxy Dragonfly 44: A Galaxy Almost Entirely Dark Matter

Even though it is relatively nearby at 300 million light years from Earth, the galaxy, named Dragonfly 44, had been missed by astronomers for decades because it is very dim. The discovery of this dim object heralds in a new class of galaxies that have become known as ‘dark galaxies’ or ‘ultra diffuse galaxies’ because they contain relatively few stars, yet have a mass equal to our own Milky Way with hundreds of billions of stars.It was only discovered recently, in 2015, when the Dragonfly Telephoto Array, then a small collection of 24 commercially available 400mm telephoto lenses, looked towards a region of sky in the constellation Coma.But as astronomers looked closer using larger telescopes with specialized instruments, they realized this dim, tiny galaxy was more than meets the eye - literally. It contained so few stars. There were so few stars in this galaxy that there was no way they could all stay together as a gravitationally bound object - they would simply fly apart. So what was keeping the stars in Dragonfly 44 together?Astronomers had pretty good idea: dark matter.Dragonfly 44 imagePieter van Dokkum / Roberto Abraham / Gemini Observatory / SDSS / AURA.