Dark matter is just that—it’s dark.
Spread across the universe, this mysterious matter doesn't interact with light, which makes it extremely difficult to detect. Scientists have yet to directly measure dark matter, but it's thought to make up around 27 percent of the universe (compared to just 5 percent made up of known matter like stars and galaxies). Without dark matter, theoretical models of our universe simply wouldn't add up.
But the idea that dark matter is a necessary ingredient for galaxies to form is being put to the test, reports Will Dunham at Reuters. Astronomers have found a distant galaxy that seems to contain no—or almost no—dark matter.
Researchers detected the sparse, see-thru galaxy called NGC1052-DF2 using the Dragonfly Telephoto Array, a New Mexico-based telescope built of camera parts that is designed to detect very faint galactic structures. They then followed up the analysis, collecting more data using the Hubble Space Telescope as well as the Gemini North and Keck Observatories in Hawaii.
Analysis shows the ultra-diffuse DF2 lies about 6.5 million light years away and is roughly the same size as our own Milky Way galaxy, but contains 200 times fewer stars. The scopes detected 10 compact groups of these stars, also known as globular clusters, within the galaxy, according to a press release from Gemini. But those clusters were moving much slower than the scientists' models predicted, suggesting that there was less mass in the system than would be expected if dark matter was present. The researchers detail their results in a study published in the journal Nature.
“If there is any dark matter at all, it's very little,” says Pieter van Dokkum of Yale, leader of the research team.“The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter.”
DF2 upends current theories about how galaxies form, which predict that the gravity of dark matter is necessary for early galaxies to hang together. “It’s like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else,” van Dokkum says of DF2 in the press release. “There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown.”
While the find may seem to refute the existence of Dark Matter—or suggest that it's unnecessary for galaxy formation—it actually may do just the opposite. The discovery of DF2 marks an important step in confirming the dark matter's existence.
As Nola Taylor Redd at Space.com reports, theories that dispute the existence of dark matter argue that the gravitational effects scientists attribute to the substance could be explained by tweaks to what we know about gravity and astrophysics. “In those theories, dark matter is not real but an illusion, caused by our lack of knowledge of gravity on large scales,” van Dokkum says. “If that's the case, every galaxy should show a dark matter signature — it’s not something you can turn on or off in those models.” But DF2 does not appear to have dark matter, which suggests that the matter not just an "illusion" or a glitch in the equation.
As Ryan F. Mandelbaum at Gizmodo reports, in 2016 the same team of astronomers found another ultra-diffuse galaxy called Dragonfly 44 that appears to be made of 99.99 percent dark matter, showing just how weird and variable these diffuse galaxies can be.
So if dark matter wasn’t present to help DF2 form, how did it come to be? As Redd reports, the researchers think it could have formed when two other galaxies merged. The diffuse DF2 could be made of gas and debris cast off during that merger. Another possibility is that interstellar winds collected enough material to help the low-mass galaxy to coalesce.
According to the press release, the team has identified 23 other diffuse galaxies, including three with properties similar to DF2, and hope to begin examining them in detail soon to tease out their secrets.
Editor's note April 2, 2018: This article has been corrected to state that DF2 is 6.5 million light years away, not 6.5 billion light years. We apologize for the error.