Dark matter is invisible and undetectable – which begs the question, how do we know it's there at all? Astronomers first suspected there was more matter than meets the eye in the 1930s, when it was observed that galaxies were moving faster than their visible mass should allow. But now there's a new wrinkle to the story, as scientists at Yale University have for the first time discovered a galaxy that appears to have no dark matter at all. Ironically, the find may just turn out to be the exception that proves the rule.

Gravity is a product of mass, so the speed at which stars and galaxies move should be proportional to their mass. But during observations of a cluster of galaxies in 1933, Swiss astrophysicist Fritz Zwicky found that wasn't the case, and calculated that there had to be far more matter in the cluster than they could see. He dubbed this mysteriously missing mass "dark matter," and scientists have been hunting for it ever since.

But that hunt has been made tricky by the fact that dark matter doesn't reflect or absorb light, and can only be indirectly detected through its gravitational interactions with ordinary (baryonic) matter. Further studies over the past 85 years have shed more light on dark matter, suggesting it clings to planets as "hairs," fills galaxies in a fuzzy, excited form, shapes the structure of galaxy clusters and connects them to each other in filaments.

Across the universe, dark matter far outweighs the normal stuff. In galaxies similar to our own Milky Way, that ratio is usually around 30 to 1, while dwarf galaxies may have up to 400 times more dark matter than ordinary matter.

That makes the galaxy NGC1052–DF2 extremely odd – it seems to have no dark matter at all. In the new study, Yale researchers measured the movements of 10 star clusters in the galaxy, and found that they were moving at about 23,000 mph (37,000 km/h). That's relatively slow, meaning that there's no "missing" mass.

"If there is any dark matter at all, it's very little," says Pieter van Dokkum, lead author of the study. "The stars in the galaxy can account for all the mass, and there doesn't seem to be any room for dark matter."

The strange galaxy came to the team's attention when it was noticed that NGC1052–DF2 looked very different through Yale's Dragonfly Telephoto Array telescope, compared to how it appears in the Sloan Digital Sky Survey data. To study the galaxy further and calculate its mass, the team followed it up with spectroscopic observations using the Keck Observatory in Hawaii, as well as the Hubble Space Telescope.

While the discovery may sound like a blow to the theory of dark matter, the researchers say it instead strengthens the case for the mysterious substance. Since dark matter was inferred to plug holes between Newton's laws of motion and what was actually observed, some scientists have suggested that the laws themselves need revision. But, the Yale researchers argue, if that were the case, then that mass discrepancy should apply to every galaxy.

The find does raise further questions of course. It was thought that dark matter and ordinary matter generally clump together in the same places in the universe, since the latter is attracted to the gravitational pull of the former. How NGC1052–DF2 formed without dark matter is uncertain – as is the origin of the galaxy Dragonfly 44, which has the exact opposite problem of being composed of 99.99 percent dark matter.

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins," says van Dokkum. "So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy."

Obviously we still have much to learn, but it's fascinating to think that one of the strongest pieces of evidence for dark matter to date is its own absence.

The research was published in the journal Nature.

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