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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. But if that's true, van Dokkum explained, then every galaxy should obey these different laws of gravity and look as if it contains dark matter, with no exceptions.

Dark matter can't be seen or touched, but it outweighs all the normal matter in the universe by more than 5 to 1.

Physicists don't know much about dark matter. In fact, it's now thought that galaxies wouldn't even form it if wasn't for dark matter; that it's dark matter that keeps the normal matter together as the galaxy coalesces, allowing the normal matter to gather into the forms we see it in. That's why they are so surprised to find a galaxy with no (or very little) dark matter. They found that dark matter is either very sparse or isn't there at all.

This Hubble Space Telescope imaging of NGC1052-DF2 was obtained 2016 November 10, using the Advanced Camera for Surveys (ACS). "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.

An image of a faint galaxy next to another image of a globular cluster in that galaxy
Gemini Observatory NSF AURA Keck Jen Miller Joy Pollard

The ghostly galaxy NGC 1052-DF2 is exceedingly diffuse. If dark matter were one of the unexplained effects of the gravity that differentiates it from regular matter, the effect of it would be visible in the galaxy. That thinking led van Dokkum and his colleagues to build the Dragonfly Telephoto Array, a telescope in New Mexico created for the express objective of scrutinizing ultra-diffuse galaxies.

"I spent an hour just staring at the Hubble image", says van Dokkum, "This thing is astonishing: a big blob that you can look through". One idea suggests it's a result of a galaxy merger, and that the gas concentrated after being ejected. From those measurements, the team calculated the galaxy's mass.

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. Instead, van Dokkum's team found those star clusters moving languidly around NGC 1052-DF2, a sign that there may well be very little or no dark matter within that galaxy at all. This is not observed in NGC 1052-DF2, which resides about 65 million light-years away in the NGC 1052 Group, which is dominated by a massive elliptical galaxy called NGC 1052. Since there's some normal matter here, any version of modified gravity would have that matter produce dark-matter-like effects. However you wouldn't expect that galaxy to be as big as this object (it's nearly the size of the Milky Way), and you'd also expect to see some other remnants around from the merger event.

Scientists spoke of "a galaxy of absolute mystery, not predicted by any theory, everything on it is odd".

A team from the Connecticut-based university used the Hubble Space Telescope to closely observe the galaxy, measuring its distance from Earth and looking at the star clusters within. But a small, distant galaxy is challenging everything we thought we knew about galaxy formation. But dark matter was the majority. However, if that was the case, DF2 would exhibit the exact same abundance of dark matter that all other galaxies - even other Ultra-Diffuse Galaxies - appear to have. The finding challenges astronomers' ideas about how galaxies come to be.



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