Joshua Sokol in Quanta:
Astronomers have long noticed an invisible elephant in the room — so-called dark matter, which seems heavier than all the visible stars, gas and dust in the cosmos by a ratio of 6-to-1.
Though no one knows what dark matter actually is, its presence has been inferred from its gravitational influence on ordinary matter in galaxy after galaxy — until now. Astronomers have found a galaxy that appears to be free of dark matter. The claim, reported today in the journal Nature, throws accepted wisdom about galaxy formation into question, and, if confirmed, may help elucidate dark matter’s true nature.
“It really is quite uncomfortable. It shouldn’t exist; it shouldn’t be real,” Marla Geha, an astronomer at Yale University who was not part of the discovery team, said of the strange galaxy. “Yet if you look at the data, that’s the conclusion you’re drawn to.”
Galaxy NGC 1052–DF2, a half-transparent smear of light 65 million light-years away in the constellation Cetus, hosts some 200 million suns’ worth of stars, and negligible amounts of gas and dust. And that’s it. According to the new study by Yale astronomer Pieter van Dokkum and colleagues, the galaxy’s visible matter accounts for its entire mass, a commonsense conclusion that turns out to be deeply strange.