Nineteen newly discovered dwarf galaxies seem to be missing their dark matter, and physicists aren’t sure why.
The find dramatically increases the number of galaxies that appear to be missing dark matter, the mysterious, invisible stuff that exerts gravitational pull, yet emits no light. Dark matter is thought to be a key ingredient in galaxy formation, with its gravity pulling together atoms of gas to form galaxies. We can tell dark matter is present in a galaxy because it makes the matter in that galaxy swirl faster than it would if the matter we see made up the galaxy’s whole mass. This faster swirling has shown up in every galaxy that could be precisely measured. Recently, however, researchers have found that certain small galaxies, now including these 19, behave as if they’re dominated by baryons — the particles that make up ordinary matter. The evidence for their unseen halos of dark matter is missing.
Kyle Oman, an astrophysicist at Durham University in the United Kingdom, who wasn’t involved in this discovery, said that these galaxies comprise the longest list of apparently dark matter-free objects yet reported. But they aren’t the first.