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Astronomers discover 'impossible' galaxy without dark matter

The observation raises questions on how galaxies are formed in the first place.

Astronomers have unveiled the first and only known galaxy without dark matter, the invisible and mysterious substance making up more than a quarter of the universe.

The observation raises questions on how galaxies are formed in the first place, according to the astronomers who have made the discovery.

The discovery of DF2 was made with a new kind of telescope developed by Roberto Abraham, an astronomer at the University of Toronto, and Pieter van Dokkum of Yale University.

Van Dokkum and his team spotted the galaxy with the Dragonfly Telephoto Array, a custom-built telescope in New Mexico they designed to find ultra-diffuse galaxies.

These faint galaxies, which include the newly discovered DF2, can be as large as the Milky Way but shine only one percent as brightly.

"For a galaxy this size, it should have 30 times as much dark matter as regular matter," he told AFP news agency on Wednesday.

"What we found is that there is no dark matter at all."

Gravitational influence

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

Dark matter is believed to be an integral part of all galaxies, the underlying scaffolding on which every galaxy is built.

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"Dark matter is a fundamental skeleton that underlies all the structure in the universe," Van Dokkum said.

So-called ordinary matter - including stars, gases, dust, planets and everything on them - accounts for only five percent of all the universe.

Dark matter and dark energy comprise the rest, and scientists have yet to directly observe either of them.

Some of the clearest evidence for the existence of dark matter comes from tracking stars in the outer regions of galaxies, which consistently appear to be orbiting faster than their escape velocity.

That is the threshold speed at which they are supposed to break free of the gravitational binds holding them in place.

This suggests that there is unseen and substantial mass holding stars in orbit.

Van Dokkum's team tracked the motions of 10 bright star clusters and found that they were travelling way below their expected velocities and looked like they were almost standing still.

The 10 clusters are unusually bright and are orbiting the nebulous DF2 galaxy.

The clusters shine far brighter than other similar objects, even though they are less dense.

The DF2 galaxy thus appears to be made up of regular matter, like stars, planets and dust.

"The stars in the galaxy can account for all the mass and there doesn't seem to be any room for dark matter," Van Dokkum said.

DF2 does not look like other galaxies. Unlike spiral galaxies, it lacks a dense central region or the signature spiral arms. And unlike elliptical galaxies, it shows no sign of a central black hole.

The ghostly, see-through galaxy is so sparse that the galaxies behind it are visible.

NASA writes that based on the colours of its globular clusters, the galaxy is about 10 billion years old.

The team is already hunting for more dark-matter deficient galaxies and are analysing Hubble images of 23 other diffuse galaxies.

Paradoxically, Van Dokkum and Abraham also argue that the discovery of a galaxy without dark matter proves that dark matter probably does exist.

Figuring out how a galaxy can be held together without dark matter will be difficult, but understanding how it formed in the first place will be even harder, Van Dokkum said.

Critics of the theory of dark matter have argued that dark matter is an illusion and the strange motions of galaxies that have been attributed to the material are the laws of gravity working differently than we initially thought.

However, if these theories are true, then all galaxies should follow the same pattern and a galaxy devoid of dark matter should not exist in the first place.

Most current narratives also suggest that dark matter dominates the universe and is at the base of the birth of galaxies. DF2, being devoid of dark matter, suggests that it was formed in a completely different way.

The findings were published on Wednesday in the scientific journal Nature.

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