There are billions of galaxies out there besides our own, so it's something of a given that the Milky Way might have a few "twins," roughly mirroring its mass and size. And now, scientists think they’ve found one, only it's like nothing like what they expected.
Called Dragonfly 44, it's about 330 million light years away and it has almost the same mass as our Milky Way. For years, it eluded detection by scientists because it has so few stars. As a team of researchers reported last week in Astrophysical Journal Letters, a whopping 99.99 percent of Dragonfly 44 is made up of dark matter.
Dragonfly 44 might effectively match the Milky Way in mass, but unlike our own galaxy, it only has one star for every 100. That's such a thin population, said the study's author Pieter van Dokkum of Yale University in a statement to Yale News, that without the dark matter to hold it in place, Dragonfly 44 "would quickly be ripped apart unless something was holding it together." It's so faint, in fact, that if they hadn't been using the ultra-powerful telescopes of the Keck and Gemini observatories in Hawaii, van Dokkum and his team may never have found it.
Technically, dark matter galaxies aren't that unknown. The key difference, though, is that most of the other known examples are so small that they would be better described as clusters. Until now, none of them have been so incomprehensibly massive as Dragonfly 44.
But it's a significant find, as it means we might be on our way to figuring out what dark matter is. As things stand, we really only know about it because of its effect on directly observable objects and phenomena. And it remains a massive question mark, as some models suggest dark matter may account for around 90 percent of the material in the universe.
"Ultimately what we really want to learn is what dark matter is,” van Dokkum said. "The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle."
from Our Galaxy Has a Twin, and It's Made Almost Entirely of Dark Matter