Dark matter accounts for almost 85 percent mass of the observable universe and yet, we have never actually found the stuff outside of theories that prove it must exist. Even the visible disk of the Milky Way, our home galaxy, is believed to exist within a roughly spherical halo of dark matter that accounts for about 90 percent of the galaxy.

In their search for the pervasive-yet-elusive particles of dark matter, astronomers have tried to find galaxies with much higher concentrations of the mysterious substance — it does not interact with visible matter at all, except through gravity, which is how scientists can theorize its existence. And while galaxies believed to be made up almost entirely of dark matter have been discovered before, they are usually quite small, such as VIRGOHI21 — about 50 million light-years away, it appears to contain no visible stars, has 99.9 percent dark matter and is about 10th the size of Milky Way.

But now, a team of astronomers has found a galaxy that is comparable in size to ours but with a similar proportion of dark matter as VIRGOHI21. Named Dragonfly 44, it is about 300 million light-years away in the Coma constellation with an estimated mass of about 1 trillion times that of the sun.

The discovery was made by astronomers from universities in the U.S. and Canada, who used the W.M. Keck Observatory and Gemini North telescope — both in Hawaii — for their observations. A paper, titled “A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44,” describing their findings was published in the journal Astrophysical Journal Letters on Thursday.

Pieter van Dokkum, an astronomer at Yale University and lead author of the paper, said in a statement: “Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together.”

The researchers measured the velocities of stars in Dragonfly 44 using data from Keck over six nights. “Star velocities are an indication of the galaxy’s mass ... The faster the stars move, the more mass its galaxy will have,” according to the statement.

The stars in Dragonfly 44 were observed to be moving much faster than was expected for a galaxy of its brightness, or rather, its lack of brightness. Roberto Abraham of the University of Toronto, co-author of the paper, said: “It means that Dragonfly 44 has a huge amount of unseen mass.”

Observations from the Gemini North telescope showed that Dragonfly 44 has “a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way galaxy.”

Abraham said the team had no idea how galaxies like this could have formed. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.” </s> But don't cue "Star Wars' " Imperial March theme music or Darth Vader breathing just yet (even if the closeup image looks like a slightly creepy emoji). Although it's massive and mysterious, Dragonfly 44 is really just misunderstood.

Dragonfly 44 went unnoticed until last year because, when regarding the darkness of space, this galaxy resembles a virtually indistinguishable blob. But by looking at it with some of the world's most powerful telescopes, including the Dragonfly telescope array designed and built by study authors Pieter van Dokkum and Roberto Abraham, researchers realized something else. It is named for the telescope that found it.

Dragonfly 44 is an incredibly large but diffuse and dim galaxy. Encircling its core is a halo made up of clusters of stars, much like what we see in the Milky Way. But this galaxy is only 0.1% stars. The Milky Way has more than a hundred times that. The researchers knew that something had to be holding those few stars in place.

"We knew as soon as we discovered the galaxy that it would be so tenuous if it was just made up of stars and no dark matter, that it would quickly disrupt and disappear," said van Dokkum, lead study author and Yale University astronomer.

A huge amount of gravity was working to hold those stars in place, and once researchers used star velocity to measure how much mass the galaxy contained, they realized that the other 99.9% is dark matter. To put this in perspective, Dragonfly 44 is comparable in size to the Milky Way, which is 100,000 light-years wide. Mostly it is just unseen because it is cloaked in darkness. "It's very exciting because we thought we had sort of figured out what the relationship is between galaxies and dark matter," van Dokkum said. "This discovery turns that on its head. Now, you can have a hundred times fewer stars in the galaxy with the same amount of dark matter as the Milky Way. That was entirely unexpected, and that means that there is something missing in our description of galaxy formations, and there are physics that we don't yet understand in that process." This newly observed galaxy could hold the secrets to understanding dark matter, the hypothesized ingredient that makes up 90% of the universe. Given the fact that we know next to nothing about it, this find could open the door to our discovery and understanding of the mysterious building block. "One of the things we are after are finding galaxies like this that are even closer to us, ideally 30 or 50 million light-years away so we could study them in detail," van Dokkum said. "We could be looking for the dark matter particle itself. Currently, people are looking at tiny dwarf galaxies that circle the Milky Way to search for the dark matter particle with X-ray and UV telescopes, but nothing has been found yet. These galaxies are a million times more massive, so we have a higher chance of detecting a dark matter signal if we find one close enough to us. "But dark matter is one of the biggest mysteries in science. We don't even know if it is a particle or not. It's a bunch of steps from where we are now. For us, it's finding the best candidates. It would be incredibly exciting if it helps the search in that way." The invention and building of the Dragonfly telescope came out of a dinner conversation between the two men, that led to a bet in 2011. It has grown from one lens in a parking lot to an array of 48 lenses with a coating that enables them to do special imaging of these faint galaxies. Abraham, a professor of astronomy at the University of Toronto, is credited with putting the telescope together. Van Dokkum also has a background in photography, which enabled him to help with selecting the lenses. As it happens, in his spare time, he uses his photography skills to capture images of the underappreciated dragonfly. He calls it a "strange confluence of work and hobby," considering the name of the galaxy and telescope. See the latest news and share your comments with CNN Health on Facebook and Twitter. Using their Dragonfly telescope, van Dokkum and Abraham will continue their research by surveying a random large batch of the sky rather than targeting of particular galaxies or groups of galaxies, as they had been doing. "We built this telescope to try to uncover what's out there," van Dokkum said. "What other things have we missed that are right above us?" </s> Dark matter and galaxies normally go hand in hand. Dark matter seems to be needed to draw in sufficient material to form the galaxy and its stars, and halos of dark matter keep galaxies from spinning apart as they rotate. So scientists were more than a bit surprised to find a galaxy that has little to no dark matter at all. Confusing things further, the galaxy appears extremely similar to others that are nearly entirely composed of dark matter.

This was one of those cases where discovery began with the phrase "huh, that looks weird." The weirdness came courtesy of the Dragonfly Array, a collection of small telescopes designed to pick up faint objects. When observing a collection of galaxies called the NGC 1052 group, the array spotted an object that had shown up in other surveys of that region of the sky.

"It stood out to us because of the remarkable contrast between its appearance in Dragonfly images and Sloan Digital Sky Survey data," the authors of the paper describing the object write. Dragonfly saw it as a diffuse object with some structures in it; Sloan imaged it as a collection of distinct objects.

Closer observations using the Hubble and 10-meter Keck telescopes showed that the object, now termed NGC1052–DF2, was a member of a recently discovered class of objects called ultra-diffuse galaxies. "This thing is astonishing," said team member Pieter van Dokkum, "a gigantic blob that you can look through. It's so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy."

Ultra-Diffuse galaxies had been identified previously in other galaxy clusters. Some of them appear to be normal galaxies that were stripped of most of their stars and gas by interactions with other members of the cluster, leaving them incredibly dark-matter rich.

To see whether this is the case with NGC1052–DF2, the researchers used the Keck telescope to image some of the structures that appeared to be present within the galaxy (the objects appeared to be bright versions of the globular clusters seen in other galaxies, including our own). Imaging them allowed the research team to determine their motions orbiting the galaxy, which could, in turn, provide a measure of the amount of mass that is involved in keeping the objects in place.

The answers were pretty surprising. The data on the 10 globular clusters the team tracked showed them moving much more slowly than would be expected. That led to an estimated mass that was extremely low for a galaxy—on the order of 108 solar masses. Using the amount of light emitted by the galaxy produced an estimate of the total mass of stars in the galaxy that was also in the neighborhood of 108. Normally, we infer that there's dark matter around because the galaxy appears to have a lot more matter than the amount provided by the stars we can see. But in this case, there's a minimal difference between the two.

In fact, the best fit for the motion of the globular clusters is a galaxy with no dark matter at all, although the uncertainties are high enough that there could be equal amounts of matter and dark matter present. But there's normally much more dark matter than ordinary stuff in most galaxies, so the authors conclude that, "NGC1052–DF2 is extremely deficient in dark matter, and a good candidate for a 'baryonic galaxy' with no dark matter at all."

The researchers seemingly anticipate the sorts of questions that will be posed by people who see this and wonder why we need any dark matter around anyway (a population that includes a number of regular Ars readers). It turns out that a galaxy without dark matter is incompatible with models that replace it using modified gravity. Since there's some normal matter here, any version of modified gravity would have that matter produce dark-matter-like effects. We don't see any indication of these effects, so modified gravity ideas must be wrong.

"Paradoxically, the existence of NGC1052–DF2 may falsify alternatives to dark matter," the authors conclude, noting that those alternatives include both variations of MOND and emergent gravity.

The obvious question is how you end up with a galaxy like NGC1052–DF2. More generally, how do you form a galaxy without dark matter? One idea is that the gas got concentrated as it was being ejected from a galaxy merger; another is that it formed from matter spewed out by quasars. The final option the authors suggest is that it formed as a large, nearby galaxy drew in a tremendous amount of gas, creating star-forming eddies.

But each of these would have to be examined in detail to determine if it could form the distinct features of NGC1052–DF2. "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," noted van Dokkum.

The researchers suggest that the somewhat-odd appearance of the globular clusters is probably related to the galaxy's unexpected properties, and they announced they're working on a paper that will describe those. They also say that they've identified a number of other galaxies that look fairly similar. Of course, they point out that most of the things we've found that look similar have turned out to have an excess of dark matter, so there's a lot we still need to figure out here. </s> The galaxy NGC 1015-DF2 has baffled astronomers because it contains almost no dark matter.

The galaxy NGC 1015-DF2 has baffled astronomers because it contains almost no dark matter.

Astronomers have detected for the first time a galaxy that is devoid of dark matter, the plentiful but enigmatic material that does not emit light or energy and had been considered a fundamental part of all galaxies including our own Milky Way.

The discovery, announced on Wednesday, is forcing scientists to rethink their ideas about the formation of galaxies.

"We didn't expect that this could happen," said Yale University astronomer Pieter van Dokkum, lead author of the research published in the journal Nature.

Paradoxically, the discovery of a galaxy without dark matter may actually confirm that the stuff actually exists by contradicting hypotheses advanced by dark matter doubters.

Van Dokkum said the galaxy, called NGC1052-DF2 and located about 65 million light years away from Earth, also appears to be devoid of gas and is relatively sparsely populated by stars.

It is about the same size as the Milky Way, but has roughly 250 times fewer stars: 400 million compared to the Milky Way's 100 billion stars. It is classified as an ultra-diffuse galaxy, a kind first recognised in 2015.

Dark matter, which is invisible, is thought to comprise about a quarter of the universe's combined mass and energy and about 80 per cent of its total mass, but has not been directly observed.

Scientists believe it exists based on gravitational effects it seems to exert on galaxies.

The universe's ordinary matter includes things like gas, stars, black holes and planets, not to mention shoes, umbrellas, platypuses and whatever else you might see on earth.

"Dark matter is not something that galaxies can sort of swap in or out of, like it's kind of an optional thing that galaxies sometimes have and sometimes don't," van Dokkum said.

"We really thought that this is the essence of what a galaxy is, that galaxies are built from, initially, a bunch of dark matter and that all the stars and all the planets and everything else is just a little frost on top," van Dokkum added.

The scientists spotted NGC1052-DF2 using the Dragonfly Telephoto Array, a telescope in New Mexico.

They do not know how it formed, but have some hypotheses, including the possibility that a cataclysm within NGC1052-DF2 swept away all its gas and dark matter or that a massive nearby galaxy played havoc with it.

Van Dokkum said NGC1052-DF2 is so sparse that "it is literally a see-through galaxy". </s> Dark matter is everywhere. Or, well, we thought it was. Dark matter is virtually unobservable, but scientists theorize it is vital to the creation of galaxies, and that it makes up about 80 percent of the matter in our universe. And now a recent discovery from a Yale-led research team may help dramatically expand our evidence in favor of dark matter, which may sound a little strange, because its game-changing discovery is a galaxy that doesn't seem to have dark matter at all.

The team, led by Pieter van Dokkum, Yale’s Sol Goldman Family Professor of Astronomy, published its report for the study, called "A galaxy lacking dark matter," in the journal Nature Mar. 28. Van Dokkum told Yale student newspaper YaleNews that by discovering a galaxy lacking dark matter, which he described as an "invisible, mysterious substance," scientists may be able to confirm dark matter exists in other galaxies.

It was van Dokkum who created and built, along with study co-author Roberto Abraham of the University of Toronto, the Dragonfly Telephoto Array, the telescope the team used to locate the dark matter-less galaxy, according to YaleNews. Called NGC 1052-DF2, the galaxy "had been cataloged previously, but the researchers noticed it looked quite different in Dragonfly images," YaleNews reported. According to Space.com, van Dokkum previously led a team that discovered the galaxy Dragonfly 44, "which is composed almost entirely of dark matter."

Van Dokkum told YaleNews NGC 1052-DF2 is a "see-through galaxy," saying, "This thing is astonishing, a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it."

Study co-author Shany Danieli, a Yale graduate student, told YaleNews, "It looked like a diffuse blob sprinkled with very compact star clusters." Danieli credited the Dragonfly for allowing the researchers to see "faint structures that no one has even seen before."

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins," van Dokkum explained to YaleNews. "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."

One reason researchers on the study concluded NGC 1052-DF2 lacks dark matter is because stars within the cluster are moving at less than 23,000 miles per hour — three times slower than stars move in galaxies containing dark matter, YaleNews reported.

Researchers also calculated NGC 1052-DF2's mass using motion measurements, and those measurements indicate that "[i]f there is any dark matter at all, it's very little," van Dokkum told YaleNews. "The stars in [NGC 1052-DF2] can account for all the mass, and there doesn't seem to be any room for dark matter."

The study's findings may also show that dark matter may not be present in the creation of all galaxies, which means it's possible there's more than one way for galaxies to form, van Dokkum added.

If the research from van Dokkum's team does end up proving dark matter exists, it will be the conclusion of a search that began in the early 1900s, according to Phys.org. The site reported on a study published in November 2017, which theorized that dark matter does not exist after researcher André Maeder, honorary professor in the Department of Astronomy in UNIGE's Faculty of Science, found that his mathematical model of the universe "predicts the accelerated expansion of the universe without having to factor in dark energy."

Whether NGC 1052-DF2 ends up leading us to a conclusion on dark matter or not, it's certainly a reminder that the truth is definitely out there. </s> WASHINGTON — It’s a double cosmic conundrum: Lots of stuff that was already invisible has gone missing.

Astronomers have found a distant galaxy where there is no dark matter.

Dark matter is called “dark” because it can’t be seen. It is the mysterious and invisible skeleton of the universe that scientists figure makes up about 27 percent of the cosmos. Scientists only know dark matter exists because they can observe how it pushes and pulls things they can see, like stars.

It’s supposed to be everywhere.

But Yale University astronomer Pieter van Dokkum and colleagues spied a vast, old galaxy with relatively few stars where what you see truly is what you get. The galaxy’s stars are speeding around with no apparent influence from dark matter, according to a study published in Wednesday’s journal Nature .

Instead of shaking the very foundations of physics, scientists say this absence of dark matter may help prove the existence of, wait for it, dark matter.

“Not sure what to make of it, but it is definitely intriguing,” wrote Case Western Reserve astronomer Stacy McGaugh, who was not part of the study, in an email. “This is a weird galaxy.”

Van Dokkum studies diffuse galaxies, ones that cover enormous areas but have relatively few stars. To look for them he and colleagues built their own makeshift telescope out of 48 telephoto lenses that he first tested by using a toy flashlight to shine a light on a paper clip. The bug-eyed telescope, called Dragonfly , peers into the sky from New Mexico.

Using Dragonfly, van Dokkum and colleagues found a large, sparse galaxy called NGC1052-DF2 in the northern constellation Cetus, also known as the whale. It’s as big as the Milky Way but with only one percent of its stars. Then they used larger telescopes on Hawaii and eventually the Hubble Space Telescope to study the galaxy.

Even though the galaxy is mostly empty, they found clusters of densely grouped stars. With measurements from the telescopes, van Dokkum and colleagues calculated how fast those clusters moved. If there were a normal amount of dark matter those clusters would be speeding around at about 67,000 mph. Instead, the clusters were moving at about 18,000 mph. That’s about how fast they would move if there were no dark matter at all, van Dokkum said.

The team also calculated the total mass of the galaxy and found the stars account for everything, with little or no room left for dark matter.

“I find this unlikely in all possible contexts,” said McGaugh, who is a proponent of a “modified gravity” theory that excludes the existence of dark matter altogether. “That doesn’t make it wrong, just really weird.”

How could this absence of dark matter help prove that it exists? By potentially disproving modified gravity theories that suggest gravity acts in a way that the cosmos makes sense without dark matter. But those alternative theories require stars in this galaxy to zip at least twice as fast as they were seen moving in this study.

Other outside scientists said the initial look at the calculations appear to be correct, though the results are confounding. A galaxy with so few stars should have more dark matter than others, not none.

“These are very strong scientists and so I take the results very seriously,” said Marc Kamionkowski, a physicist at Johns Hopkins University.

One outsider suggested that perhaps the “galaxy” van Dokkum studied is so diffuse that it may not really be a galaxy. Another suggested that the dark matter might just be outside of the area that van Dokkum measured.

Van Dokkum dismissed both possibilities. “It’s sort of non-negotiable. There’s nothing else, just the stars,” he said. The only way this can be explained is if dark matter exists in the universe, just not in that galaxy, he said.

There’s no good explanation for why and how this galaxy has no dark matter, van Dokkum said. He proposed four different possibilities — all unproven. His favorite: That the galaxy formed in the very early universe in a way astronomers have never seen or understood.

“It’s not so often you get a true surprise,” van Dokkum said. </s> Galaxy NGC1052-DF2 is so transparent that you can see galaxies behind it.

It may not look like much, but this giant, fuzzy blob of a galaxy has astronomers perplexed.

The galaxy, called NGC 1052-DF2, or DF2 for short, contains virtually no dark matter.

"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of galaxies," lead author Pieter van Dokkum of Yale University said.

He said the discovery, reported in the journal Nature, raised questions about how galaxies formed and the role of dark matter.

It is thought that blobs of dark matter in the early universe attracted hydrogen gas, which coalesced to form the first stars and galaxies not long after the big bang.

"The idea is that dark matter is not just an object inside a galaxy but it helps form the galaxy," Professor van Dokkum explained.

Professor van Dokkum argues the oddball galaxy appears to defy existing explanations for how galaxies are formed.

But not all astronomers are convinced the discovery is a game-changer.

DF2, which was first discovered several years ago, lies 65 million light years away in the constellation of Cetus the whale.

A lack of dark matter is not its only unusual feature, Professor van Dokkum said.

It also appears to have a very low mass.

"It's about the size of the Milky Way … but it has very few stars. It has about 200 times less stars than the Milky Way," Professor van Dokkum said.

To get an idea of its mass, the researchers faced a challenge.

Because the stars are spread out across a large area this type of galaxy, known as an ultra-diffuse galaxy, is faint and hard to detect using standard telescopes.

So Professor Dokkum and colleagues used a custom-built telescope called the Dragonfly Telephoto Array to peer into the guts of this ghostly galaxy.

What appeared as tiny specks of light on other surveys of the sky appeared as a big bright blob.

They discovered the galaxy contains exceptionally bright spheres of stars known as globular clusters.

By using the Keck Telescope to study how fast 10 of these clusters were moving, the researchers were able to get an idea of the mass of the galaxy.

"If they move fast then there's a lot of mass holding things together, and if they move slowly then there's not a lot of mass holding things together," Professor van Dokkum said.

The researcher calculated that the galaxy had a total mass of no more than 350 million solar masses, and that could be attributed to the stars, gas and dust, leaving little room for any dark matter.

But galaxies with a low mass like this are usually dominated by dark matter.

"This galaxy should have 400 times more dark matter than luminous matter," Professor van Dokkum said.

Follow-up observations using the Hubble Space Telescope revealed even more unusual features of DF2.

"Normally a galaxy has spiral arms or it's a big elliptical galaxy, but this thing is barely there," he said.

What does this tell us about galaxies?

Professor van Dokkum said he "had no idea" what DF2 tells us about galaxy formation.

"In the paper we provided a few explanations for [how the galaxy could have formed] but … none of them really describe this object very well," he said.

"So we're kind of at a loss by this find."

But Ken Freeman, an Australian astronomer known for his work in dark matter and evolution, argues that galaxies like DF2 with no dark matter are unusual, but not unique — and there is a reasonable explanation for how it formed.

"Dark matter is pretty much everywhere, but occasionally you do get galaxies that have been made by some other route," Professor Freeman said.

"So I don't think [the discovery] is going to change anyone's view on what's going on out there."

Professor Freeman said DF2 could be a type of galaxy called a tidal dwarf.

"They basically don't have any dark matter at all because it's just ordinary matter that's been ripped out of a galaxy," Professor Freeman, who is based at the Australian National University, said.

DF2 could, for example, have formed from a nearby giant elliptical galaxy called NGC 1052.

Professor van Dokkum argues this was one of the hypotheses the researchers explored, but there were still aspects of the galaxy that didn't fit.

"In this case there's just no evidence for interaction of this thing with anything else or past interaction where you could see tidal leftovers," he said.

The stars themselves also didn't resemble stars from a galaxy with a larger mass, he added.

Other possibilities Professor van Dokkum and his colleagues considered are that the galaxy was formed from gas swept up by quasar winds or that it was gas that never made it to its larger neighbour.

Professor Freeman said it was also possible — although less likely than the tidal galaxy hypothesis — that DF2 does have dark matter, but it had not yet been detected.

He said DF2 could be more like a large mass galaxy like our own Milky Way, where the dark matter dominates the outer edges.

"We would have no way of knowing that dark matter is there because there is no data on the motions far out in the galaxy.

"Only when we were able to measure motions in the outer parts of the [Milky Way], could we see how dominant the dark matter really is.

"That is why dark matter in large galaxies was not discovered until the 1970s."

Where to next?

Professor van Dokkum said the next step would be to use the Keck telescope again to try to pin down the mass of the galaxy more precisely.

"All we have right now [using the velocity of clusters] is an upper limit," he said.

"[Getting that measurement] will be very difficult … but we think it's worth it given how interesting this object has turned out to be."

The team would also like to pin down the age of the galaxy using the Hubble telescope, which in turn could give them clues as to how the galaxy formed.

"At the moment we only know its older than 10 billion years, but we'd like to know if it's 10 billion years old or 13 billion years old, which is right after the big bang," Professor van Dokkum said.

If the galaxy was from 10 billion years ago, it would have formed at the same time as other galaxies in that group and it would be possible it could be gas that was "side tracked" from a bigger galaxy next door.

"If it's 13 then … it's a whole different story."

The team also plans to continue the hunt for more galaxies like DF2 using the Dragonfly telescope. </s> Astrophysicists have discovered a galaxy that contains very little dark matter, which funnily enough, might help to prove that dark matter exists.

A team led by researchers from Yale University noticed the galaxy, NGC 1052-DF2, because it looked rather strange in images from the Dragonfly Telephoto Array.

To get a better picture, the team used the WM Keck Observatory in Hawaii, NASA’s Hubble Space Telescope and the Gemini Observatory in Hawaii. What they found utterly defied expectations.

“I spent an hour just staring at the Hubble image,” said Pieter van Dokkum, Yale’s Sol Goldman Family Professor of Astronomy and lead author of a new study in the journal Nature.

“ It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and say, ‘I’ve never seen that before’ . This thing is astonishing, a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”

We thought that every galaxy had dark matter and that dark matter is how a galaxy begins. This invisible, mysterious substance is the most dominant aspect of any galaxy, 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.”

Dark matter cannot currently be “seen” by us, the only thing that proves it exists is its gravitational interactions with normal matter. Nevertheless, it is believed to make up around 80 per cent of the mass of the Universe and many theories also suggest that dark matter is integral in the formation of galaxies. </s> A distant galaxy that appears completely devoid of dark matter has baffled astronomers and deepened the mystery of the universe’s most elusive substance.

The absence of dark matter from a small patch of sky might appear to be a non-problem, given that astronomers have never directly observed dark matter anywhere. However, most current theories of the universe suggest that everywhere that ordinary matter is found, dark matter ought to be lurking too, making the newly observed galaxy an odd exception.

“Something like this has never been seen,” said Prof Pieter van Dokkum, of Yale University, the study’s senior author. “It challenges the standard ideas of how we think galaxies form.”

Dark matter’s existence is inferred from its gravitational influence on visible objects, which suggests it dominates over ordinary matter by a ratio of 5:1.

Some of the clearest evidence comes from tracking stars in the outer regions of galaxies, which consistently appear to be orbiting faster than their escape velocity, the threshold speed at which they ought to break free of the gravitational binds holding them in place and slingshot into space. This suggests there is unseen, but substantial, mass holding stars in orbit.

In the Milky Way there is about 30 times more dark matter than normal matter. The latest observations focused on an ultra-diffuse galaxy – ghostly galaxies that are large but have hardly any stars – called NGC 1052-DF2.



The team tracked the motions of 10 bright star clusters and found that they were travelling way below the velocities expected. “They basically look like they’re standing still,” said van Dokkum.

The velocities gave an upper estimate for the galactic mass of 400 times lower than expected. “If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all of the mass, and there doesn’t seem to be any room for dark matter.”

Paradoxically, the authors said the discovery of a galaxy without dark matter counts as evidence that it probably does exist. A competing explanation for the fast-orbiting stars is that the way gravity drops off with distance has been misunderstood – but if this were the case, all galaxies should follow the same pattern.

Andrew Pontzen, a cosmologist at University College London who was not involved in the work, said: “Alternative gravity theories tend to be tuned to reproduce typical galaxies, which means they can struggle to account for anything new or unusual.”

The observation also raises questions about how galaxies formed in the first place. Most current narratives suggest that because dark matter dominates the universe, slightly denser patches of dark matter would have provided the initial seeds for things to clump together in the early universe and eventually end up as stars and galaxies. “This whole story is one of dark matter being the scaffolding on which every galaxy is built,” said van Dokkum. “It’s not just a component of a galaxy, like a spiral arm, it’s more like the fundamental skeleton that underlies all the structure in the universe.”

Prof Jeremiah Ostriker, a professor of astronomy at Columbia University in New York, described the observation as highly significant. “The standard paradigm has gas falling into halos – or lumps – of dark matter and turning into stars to make a galaxy,” he said. “This one was clearly made some other way.”

Speculative explanations include that a collision or cataclysmic event within the galaxy resulted in all the dark matter being swept away.

Van Dokkum and colleagues identified the galaxy, NGC 1052-DF2, using a low-budget setup called the Dragonfly Telephoto Array in New Mexico, which they designed from 48 commercial cameras and paparazzi-style lenses.

The initial images just showed a ghostly blob on the night sky, but by peering more closely at it using the Gemini Multi Object Spectrograph and Keck telescopes they were able to pick out star clusters within the galaxy and track their movements. The team are now turning to look at other ultra-diffuse galaxies to see whether any others are similarly deficient in dark matter.

The findings are published in the journal Nature.
A newly-discovered galaxy known as Dragonfly 44 appears to be made up mostly of dark matter.