spaceSpace and Physics

Astronomers Have Found A Galaxy That Is Made Almost Entirely Of Dark Matter


Jonathan O'Callaghan

Senior Staff Writer

A distant view (left) and close-up (right) of the galaxy. Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA

Astronomers have found a galaxy that they think might be made of 99.99 percent dark matter. Called Dragonfly 44, it is located more than 300 million light-years from Earth in the Coma constellation, but is strangely devoid of “normal” matter – leading astronomers to their conclusion.

The galaxy is described in a new paper published in the Astrophysical Journal Letters. It has a similar mass to our own Milky Way galaxy, about 1 trillion times the mass of the Sun. But stars, and other such normal matter, only account for 0.01 percent of the mass of the galaxy. In fact, it has 100 times fewer stars than the Milky Way.


Smaller galaxies have been found with a similar composition, typically ultra-faint dwarf galaxies about 10,000 times less massive. But one of this size is a rare find.

“Very soon after its discovery, we realized this galaxy had to be more than meets the eye,” said Yale University astronomer Pieter van Dokkum, who led the study, in a statement. “It has so few stars that it would quickly be ripped apart unless something was holding it together.”

The galaxy was studied in detail using the Gemini North telescope and the W. M. Keck Observatory on Mauna Kea, Hawaii, having previously been seen by the Sloan Digital Sky Survey. Considering its size, the galaxy is unusually faint, making it appear as a dim smudge, rather than something like our own Milky Way.

But how it came to be in this situation is not known. “We have no idea how galaxies like Dragonfly 44 could have formed,” co-author Roberto Abraham from the University of Toronto added in the statement. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”


Dark matter is thought to make up more than 90 percent of the universe, but we’ve yet to actually observe it directly. Objects like this, though, give us a pretty good indication that it is definitely out there; without it, this galaxy would simply drift apart.

Finding more galaxies like this, and perhaps some that are closer to home, could let us move closer to actually finding a dark matter particle.


spaceSpace and Physics
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