Image copyright NASA/ESA/P. van Dokkum Image caption Hubble image: The galaxy is so faint that other spiral galaxies can been seen through it

An unusually transparent galaxy about the size of the Milky Way is prompting new questions for astrophysicists.

The object, with the catchy moniker of NGC1052-DF2, appears to contain no dark matter.

If this turns out to be true, it may be the first galaxy of its kind - made up only of ordinary matter. Currently, dark matter is thought to be essential to the fabric of the Universe as we understand it.

The study is published in Nature.

Image copyright Science Photo Library Image caption Artwork: A dark matter halo (blue) is thought to extend beyond the visible star population of galaxies

Ghostly glow

The authors of the study weren't initially on the hunt for a dark matter-free galaxy; instead they had set out to take a closer look at large, ultra-diffuse galaxies.

These are similar in size to the spiral galaxies we're more familiar with, but have a fraction of the number of stars.

When Prof Pieter van Dokkum, lead author of the study, first spotted NGC1052-DF2, "I stared a lot at that image and just marvelled at it... It's like this ghostly glow in the sky."

The galaxy has very few stars, but many of them are grouped together in unusually bright clusters. When the team studied the behaviour of these clusters, they found that the stars seemed to account for all of the galaxy's mass.

Leaving no room for dark matter.

Image copyright Science Photo Library Image caption Artwork: a map of dark matter in the local universe

This is not the case for most galaxies.

"There's about five times more dark matter in a galaxy than regular matter," explained Dr Michelle Collins, a physicist at the University of Surrey who was not involved in the study.

"As you go further out from the galaxy you have fewer stars and more dark matter. The dark matter halo is much more extended than the stars are in a galaxy," she added.

You can't have one without the other

As it has greater mass than normal matter, dark matter is believed to hold the necessary gas together while galaxies are forming.

"So this galaxy would have to form a different way: maybe from interactions within gas that's flowing into or blowing out of a larger galaxy," North Carolina State University astrophysicist Dr Katherine Mack told BBC News.

Image copyright Science Photo Library Image caption Andromeda: most galaxies have five times more dark matter than regular matter

"It's not just galaxies," explains van Dokkum. "The entire fabric of the universe is really the scaffolding of dark matter and everything else is pasted on it."

For Mack, the most exciting aspect of this galaxy is its potential to prove that dark matter - until now widely theorised but not directly observed - is real.

If dark matter were just an unexplained effect of the gravity from regular matter, its effects would be visible in this galaxy. "So it only makes sense if dark matter is a real substance, that can be present or not, separately from the regular matter," Mack added.

The team has another paper forthcoming that will take a closer look at the bright star clusters, and may unravel more of NGC1052-DF2's mystery.

Dark materials

More work remains to be done on this and similar objects before dark matter theory needs to be fundamentally altered, however.

Image copyright Getty Images Image caption Once launched, the James Webb Space Telescope will offer a better look at distant galaxies

"You always have to be careful to say it's the first of anything," says van Dokkum. "It's certainly the best candidate for a baryonic [ordinary matter] galaxy."

Dr Collins is cautious to conclude that the galaxy has no dark matter halo, based on the current evidence.

She notes that there may be other galaxies with unusual and unexplained features, but technology will need to improve before they can be properly observed.

"We have some galaxies nearby that show some similar properties but they're much fainter. They're much smaller objects," she told BBC News.

Dr Richard Massey, a physicist at Durham University, agrees: "I'm genuinely very impressed with the work, and I'd use the conclusions to say that we should stare at these objects a lot harder for a lot longer - but I wouldn't conclude anything profound about dark matter quite yet," he told BBC News.</s>Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M halo /M stars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses1,2. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies3,4. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy5 NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio M halo /M stars is of order unity (and consistent with zero), a factor of at least 400 lower than expected2. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.</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>In a shocking discovery, astronomers have found a galaxy that is missing most — if not all — of its dark matter. This discovery of the galaxy NGC 1052-DF2, detailed in the journal Nature, challenges currently-accepted theories of and galaxy formation and provides new insights into the nature of dark matter.

“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built,” explained study co-author Allison Merritt from Yale University in the US and the Max Planck Institute for Astronomy, Germany.

“This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work,” said lead researcher Pieter van Dokkum of Yale University. “There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown,” Merritt said.

For the study, the researchers used the NASA/European Space Agency’s Hubble Space Telescope and several other observatories. Hubble helped to accurately confirm the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Based on these data the team discovered that the newly discovered galaxy is larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.

“I spent an hour just staring at this image,” van Dokkum said as he recalled first seeing the Hubble image of NGC 1052-DF2. “This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy,” he added. Further measurements of the dynamical properties of 10 globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxies mass.

This mass is comparable to the mass of the stars in the galaxy, leading to the conclusion that NGC 1052-DF2 contains at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all. This discovery is unpredicted by current theories on the distribution of dark matter and its influence on galaxy formation. The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.

Meanwhile, the researchers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2. Did a cataclysmic event such as the birth of a multitude of massive stars sweep out all the gas and dark matter? Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago play a role in NGC 1052-DF2’s dark matter deficiency?

These ideas, however, still do not explain how this galaxy formed. To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyse Hubble images of 23 ultra-diffuse galaxies — three of which appear to be similar to NGC 1052-DF2.

For all the latest Technology News, download Indian Express App</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>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.</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>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>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.
Research published in the journal Nature implies the existence of a galaxy that appears to contain no dark matter, dubbed NGC1052-DF2. If confirmed it would be the first such galaxy discovered.