Galaxies are home to vast communities of stars, some likely numbering in the hundreds of millions. But not all of these stellar neighborhoods glow with starlight. Some galaxies as wide as the Milky Way are all but dark. Unlike Andromeda and other well-known galaxies, these dark beasts have no grand spirals of stars and gas wrapped around a glowing core. Nor are they radiant balls of densely packed suns. Instead, these may amount to no more than a wisp of starlight barely illuminating some tenuous blob.
“If you took the Milky Way but threw away about 99 percent of the stars, that’s what you’d get,” says Roberto Abraham. He’s an astrophysicist in Canada at the University of Toronto. How these dark galaxies form is unclear. They could be a whole new type that challenges ideas about how such stellar communities form. Or they might be outliers of already familiar galaxies — some type of black sheep, shaped by their environment. Wherever they come from, dark galaxies appear to be everywhere. Astronomers reported the first batch in early 2015. That group showed other scientists what to look for. In no time, similar dark denizens were found inhabiting many nearby clusters of galaxies. “We’ve gone from none to suddenly over a thousand,” Abraham says. “It’s been remarkable.” This haul of ghostly galaxies has proved puzzling. Any galaxy the size of the Milky Way should have no trouble creating lots of stars — and light. But it’s still unclear how massive the dark galaxies are. Perhaps these shadowy entities are failed galaxies. Despite being as massive as our own, some unknown factors may have kept them from giving birth to a vast family of stars. Or despite being as wide as the Milky Way, our dark neighbors could be relative lightweights and stretched thin. If there is not enough tightly condensed mass, they may not have had the critical heating and density to fire up much star formation. Either way, with so few stars, dark galaxies must have enormous deposits of unseen matter. If not, the gravity of neighboring galaxies would have pulled them apart. Astronomers can’t resist a good cosmic mystery. With the number of these dark oddballs piling up, there is a push to figure out just how many exist and where they’re hiding. “There are more questions than answers,” says Remco van der Burg. He’s an astrophysicist at CEA Saclay, near Paris, France. Cracking the code of dark galaxies could help scientists better understand how all galaxies formed and evolved, including our Milky Way.