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There鈥檚 more than meets the eye. Astronomers have weighed a so-called ultra-diffuse galaxy for the first time, and found that it is over 99.96 per cent dark matter.

An ultra-diffuse galaxy can be as large as the Milky Way but as dim as a dwarf. The galaxy鈥檚 few stars are spread out, so it looks ghostly, making it hard to study.

Although observers spotted the first few examples three decades ago, they didn鈥檛 have a name until 2014, when a team led by 听at Yale University discovered 47 of them in the Coma galaxy cluster. Other astronomers studied听this cluster with the giant Subaru Telescope in Hawaii and .

Van Dokkum鈥檚 team argued that the galaxies had to consist of at least 98 per cent dark matter for gravity to hold them together. Otherwise, the many other galaxies in the Coma cluster would tear them apart.

Dark matter is thought to make up about 80 per cent of the mass in the universe overall, so that would be an impressively dense concentration of the stuff in a small space. But until now no one had directly measured an ultra-diffuse galaxy鈥檚 mass.

Now 听at听the Institute of Astrophysics of the Canary Islands, Spain, and his colleagues have weighed an ultra-diffuse galaxy in the Virgo cluster named VCC 1287. The galaxy鈥檚 main body is too dim to study easily, so instead they observed seven of its globular clusters 鈥� bright, tight-packed gatherings of stars that move around the galaxy.

Dim but heavy

Using the 10.4-metre in La Palma, Beasley鈥檚 team measured the clusters鈥� speeds. They found that the clusters orbit the galaxy so fast that it must have about 80 billion times more mass than the sun. That鈥檚 only 8 per cent of the Milky Way鈥檚 total mass, but an impressive figure for a galaxy that has so few stars that it emits less than a thousandth as much light.

It also means the galaxy has 3000 times more dark matter than stellar mass. The Milky Way鈥檚 ratio is just 15 to 1 .

鈥淚t鈥檚 a very clever method,鈥� says van Dokkum. 鈥淚t鈥檚 a great achievement.鈥�

Beasley speculates that the galaxy he studied was born with both dark matter and gas but lost the latter as the galaxy fell into the Virgo cluster. Without gas, the galaxy couldn鈥檛 create new stars, so it ended up with lots of dark matter but little light.

Journal reference: The Astrophysical Journal Letters, DOI:

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