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

Key points: Astronomers studied a very faint galaxy located 65 million light years away

Astronomers studied a very faint galaxy located 65 million light years away Low mass galaxies like this tend to have high concentrations of dark matter but this one has virtually none

Low mass galaxies like this tend to have high concentrations of dark matter but this one has virtually none The oddball galaxy challenges current theories about galaxy evolution and the role of dark matter, the astronomers say

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.



"[Dark matter] is the scaffolding on which galaxies are built … Not seeing it means that something else happened."

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.

A very unusual galaxy

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.

The Dragonfly telescope uses 48 telephoto lenses to identify faint objects in the sky. ( Supplied: University of Toronto )

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.

"These luminous star clusters, this giant blob of a galaxy, and the lack of dark matter is really something that we really haven't seen before," Professor van Dokkum said.

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 researchers calculated that the galaxy had a total mass of no more than 350 million solar masses, and most of that could be attributed to the stars, 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.

"Seeing this huge swathe of stars you can look straight through to the rest of the universe beyond was a very special moment."

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 galaxy evolution, argues that objects 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.

The astronomers studied 10 exceptionally luminous globular clusters, which were almost as bright as Omega Centauri (pictured). ( Supplied: ESO )

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.