Mumbai, November 2: Roughly 65 million light-years away from Earth is a galaxy called NGC 1052-DF2 (DF2 for short). But DF2 may as well be called F-U, because that’s what it’s saying to scientists who thought they understood galaxies, dark matter, and really anything about our universe.
We’ve never seen dark matter directly. We only believe dark matter exists because we can see how it affects “regular,” or baryonic, matter. Based on these indirect observations, researchers have estimated that dark matter makes up about 27 percent of our universe.
Since dark matter was (sort of) discovered, researchers assumed dark matter was essential to galaxy formation. Dark matter would clump together. Then, the gravity from those clumps would attract baryonic matter, forming the stars, planets, and other objects we can actually see within a galaxy.
Based on this understanding, the team studying DF2 thought they had a pretty good idea how much dark matter it contained. But when they calculated how much dark matter DF2 actually had, they discovered it contained only 1/400th the amount they expected.
“It challenges the standard ideas of how we think galaxies work,” Pieter van Dokkum, a Yale University professor and lead author of a paper on DF2, now published in Nature, said in a press release. “This result also suggests that there may be more than one way to form a galaxy.”
DF2 is unique in other ways, too. It doesn’t fit the characteristics of a spiral galaxy, which typically have dense, central regions, spiral arms, and a disk. But it also isn’t like known elliptical galaxies, which have a black hole at their center.
Instead, DF2 is a rare ultra-diffuse galaxy. “It’s so sparse that you see all of the galaxies behind it,” van Dokkum said. “It is literally a see-through galaxy.”