B ack in 2016, researchers claimed to have found a galaxy made almost completely of dark matter. Now that claim has fallen apart. The galaxy, Dragonfly 44 (DF44), belongs to a class of mysterious objects known as ultra-diffuse galaxies, or UDGs. Researchers have debated since the 1980s whether these vast, dim objects have a low mass, like dwarf galaxies smeared across huge reaches of space, or are heavy, Milky Way-style galaxies that seem dim for two reasons: because they have almost no stars, and because a huge fraction of their mass is dark matter found in the outer fringes of the galaxy in so-called dark matter haloes that emit no light. In a paper published in 2016 in the Astrophysical Journal Letters, scientists argued that DF44 was one of these galaxies with a big dark matter halo and few stars. They estimated its mass and found it was at least 98 per cent dark matter.
But a new analysis suggests the earlier study got it wrong. Researchers in the 2016 study assumed a bunch of mass was globbed into the dark matter halo, but the new study showed a much lower total mass, indicating DF44 is one of those low-mass dwarf galaxies spread across space with normal percentages of dark matter. DF44 is about 360 million light years from Earth, so astronomers can’t directly measure its mass. Instead they rely on proxies. Features like the speed at which objects circle a galaxy can indicate how massive it really is, as more gravity would cause objects to whirl faster.