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Astronomers Discover Hundreds of Weird Galaxies Filled With Dark Matter

Astronomers say they've detected 854 "ultra-dark galaxies" that consist almost entirely of dark matter — and that's just the tip of the iceberg.
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Last year, astronomers were surprised to detect 47 galaxies in the Coma Cluster that were made almost entirely of dark matter. So how much more surprised are they to see 800 more dark galaxies in the same cluster?

Even that many concentrations of mysterious dark matter may be merely the "tip of the iceberg," said Jin Koda, an astrophysicist at Stony Brook University in New York.

"We may find more if we look for fainter galaxies embedded in a large amount of dark matter," Koda said in a news release about the latest find. The discovery, detailed in the June issue of the Astrophysical Journal Letters, is based on observations from the 8.2-meter (27-foot) Subaru Telescope in Hawaii.

Image: Dark galaxies
This image shows a small region of the Coma Cluster that was surveyed by the Subaru Telescope. Yellow circles highlight two of the 47 dark galaxies discovered last year. The green circles highlight just some of the 854 dark galaxies reported in the latest research paper.NAOJ

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Koda and his colleagues documented 854 ultra-dark galaxies by sifting through Subaru's archives and finding diffuse and extended galaxies in the Coma Cluster, which is around 300 million light-years away in the constellation Coma Berenices. These dim galaxies are arranged in a pattern similar to their brighter cousins in the Coma Cluster, and many of them are similar in size to our own Milky Way — but they have only a thousandth as many stars.

The tidal forces at work in the galaxy cluster should have torn such sparse galaxies apart, and yet they survive. Why? The astronomers say the stars must be sheathed in protective concentrations of dark matter. Their calculations suggest that dark matter accounts for more than 99 percent of the galaxies' mass.

Flash Interactive: Why Does Dark Matter Matter?

These ultra-dark galaxies apparently lost most of the gas they needed to create stars — perhaps due to the violent pressures resulting from gravitational encounters and supernova explosions within the Coma Cluster. "Follow-up spectroscopic observations in the future may reveal the history of star formation in these dark galaxies," Koda said. And that, in turn, may provide new insights for models of galaxy formation.

But what is dark matter? Although the stuff is thought to account for 80 percent of the universe's mass, no one really knows what it is. So far, it's been detectable only by its gravitational influence on galaxies like those in the Coma Cluster. Solving the mystery is high on the list for researchers working with instruments ranging from the Large Hadron Collider to the Alpha Magnetic Spectrometer and the Fermi Gamma-Ray Telescope.

In addition to Koda, the authors of "Approximately a Thousand Ultra Diffuse Galaxies in the Coma Cluster" include Masafumi Yagi, Hitomi Yamanoi and Yutaka Komiyama.