Ghostly galaxy without dark matter stuns astronomers

In a first, scientists have discovered a galaxy that is nearly completely devoid of dark matter - the mysterious substance believed to make up most of the universe.The galaxy, known as NGC1052-DF2, has been classified as an an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015.

The first anomalies discovered were galaxies nearly entirely composed of dark matter. It's paradoxical, but the galaxy lacking dark matter could teach us a lot about dark matter itself. The galaxy was found to have very few stars with many of them grouped together in unusually bright clusters.

"This invisible, mysterious substance is the most dominant aspect of any galaxy", Pieter van Dokkum, a Yale professor and lead author of the study, explains.

Figuring out how something as big as a galaxy is held together without dark matter will be difficult, but understanding how it formed in the first place will be even harder, he said. His stars are enough to explain his whole mass, and there seems to be no extra space for dark matter, "he added". This galaxy without dark matter has never been observed before and it brings confusion to everything we've known about this invisible and odd thing that is so present in the cosmos. On the other hand, he said, "this shows that dark matter is separate from galaxies". It's like finding a body without a skeleton. The visible matter accounted for almost all of the total mass of the galaxy, which had never been seen before.

However, it's still too early to throw out the old rules, says astrophysicist James Bullock of the University of California, Irvine. It is located about 65 million light-years away in the constellation of Cetus.

The team is already hunting for more dark-matter deficient galaxies. Had there been dark matter, the clusters would be moving slower or faster. In fact, there's so little mass that the researchers have come to the astonishing conclusion that there's little, if any, dark matter there.

But that doesn't mean the results in this paper provide ideal support for dark matter theories either.

Dark matter is believed to exist because of its gravitational effects on visible matter in the cosmos.

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The next step is to figure out whether this galaxy is an exception or the norm, says Ostriker. "Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work", van Dokkum said.

We don't really know what dark matter is and why it exists... or why it doesn't. Similarly, MOND theorists expect to find the most significant observable effects of their modified laws in less massive galaxies like NGC 1052-DF2, and lesser effects in more-massive galaxies.

"We asked ourselves where we had screwed up, if the measurements were wrong", he said.

Follow-up observations with the Hubble Space Telescope revealed DF2's other unusual qualities. LUX-ZEPLIN and XENON1T both use liquid xenon to hunt for WIMPs.

"I spent an hour just staring at the Hubble image", says van Dokkum.

Therefore, researchers were surprised when they uncovered a galaxy that is missing most, if not all, of its dark matter. Recently, the space scientists unveiled a certain ghostly galaxy that is very unusually transparent and is nearly the same size as the Milky Way.

They're hoping that these weird objects will help guide theorists like Ostriker and Bullock to better understand what dark matter is.

"If there is any dark matter at all, it's very little", van Dokkum explained.