Galaxy found that is 99.99% dark matter

The exact nature of the elusive substance known as dark matter remains a mystery, but observations of massive cosmic structures may hold the key. The latest discovery is a galaxy that consists almost entirely of dark matter.

An image of Dragonfly 44 taken with the Gemini North telescope. One the right is a closeup of the galaxy, showing its elongated shape and halo of spherical clusters.
Image Credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA

A galaxy has been discovered that is made up of 99.99 per cent dark matter.

The galaxy, named Dragonfly 44, is estimated to be a trillion times the mass of the Sun, yet only one hundredth of one percent of it consists of stars and 'normal' matter. In fact, even though the galaxy’s mass is similar to the mass of the Milky Way, the Milky Way has over a hundred times more stars.

Astronomers had missed spotting the galaxy for decades because it is very dim: much dimmer than it should be, considering its mass. It was discovered last year as a result of observations by the Dragonfly Telephoto Array of the sky in the constellation Coma Berenices.

When the team looked at the galaxy closely, they realised that it did not contain enough visible mass to create enough gravity to hold the structure together. Observations such as this have led astronomers to conceive of the concept of dark matter: i.e. matter that cannot be seen or detected with telescopes, but which must exist in order to explain how large cosmic structures are held together. Dark matter has never been directly detected.

The team behind the study used the DEIMOS instrument on Keck II to observe the velocities of stars over 33.5 hours to determine the galaxy’s mass, while the Gemini North telescope on Maunakea in Hawaii revealed a ring of spherical star clusters around the galaxy’s core.

Peter van Dokkum of Yale University, who led the study, explains: “Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, than there is mass in the stars themselves.” 

"We have no idea how galaxies like Dragonfly 44 could have formed,” says study co-author Roberto Abraham. "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."


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