In a world first, astronomers have found a galaxy that lacks the enigmatic substance known as dark matter – long considered one of the universe’s fundamental building blocks.

Its discovery challenges well-established ideas about how galaxies form, and the nature of dark matter itself.

Located 65 million light years away, the snappily named NGC 1052-DF2 galaxy – or DF2 for short – is a “complete mystery” according to the scientists who found it.

While dark matter has yet to be directly observed by scientists, it is generally considered a vital ingredient in the birth of galaxies.

“We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating,” said Professor Roberto Abraham, an astronomer at the University of Toronto who co-authored the paper describing the discovery. 

“Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre.”

DF2 is known as an “ultra-diffuse” or “ghost” galaxy, an extremely low-density variety, recognisable due to its large size and faint appearance.

However, this one is “an oddity, even among this unusual class of galaxy”, according to Shany Danieli, a Yale University graduate student who contributed to its discovery.

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The astronomers realised something about DF2 was amiss when telescope observations revealed that 10 clusters of stars within it were moving far slower than would normally be expected.

The velocities of stars and other objects in faraway galaxies can be used to measure their individual masses. 

By performing these calculations, the research team found that all the mass in the galaxy could be attributed to the visible stars, gas and dust. There was essentially no remaining room in this galaxy for dark matter.

“If there is any dark matter at all, it’s very little,” said Professor Pieter van Dokkum of Yale University, the study’s lead author.

The analysis of this new galaxy was published in the journal Nature.

The discovery was unexpected because while dark matter remains largely mysterious, it is nevertheless considered by many to be the most dominant substance in the universe.

In the Milky Way, for example, scientists have suggested there is around 30 times more dark matter than normal matter.

Dark matter is also thought to have a hand in the birth of galaxies.

“For decades, we thought that galaxies started their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way,” said Professor Van Dokkum.

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No previous theory has predicted the discovery of a galaxy like DF2, and its discovery calls into question these fundamental ideas about galaxy formation.

“The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown,” said Professor Van Dokkum.

“This result also suggests that there may be more than one way to form a galaxy.”

Counterintuitively, Professor Van Dokkum and his colleagues suggest the lack of dark matter in DF2 is actually good evidence for its existence.

While this substance plays a central role in our understanding of the universe, its intangible nature means alternate theories have been suggested to account for the gap in scientific understanding of what is currently known as dark matter.

These theories consider the dark matter signature that astronomers measure to be an unavoidable consequence of ordinary matter.

Therefore, the existence of a galaxy that has lots of matter, but no dark matter, suggests dark matter does indeed exist elsewhere as a substance in its own right.

“This discovery shows that dark matter is real – it has its own separate existence, apart from other components of galaxies,” said Professor Van Dokkum.

The astronomers suggest that DF2’s dark matter could have been swept away by the birth of many massive stars, or the presence of a giant galaxy nearby.

However, for the time being they can only speculate about how it came to be in its current state, and they are now undertaking a survey to look for more dark matter-deficient galaxies and unravel this mystery. 

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