Following on a surprising find reported last year, astronomers have now discovered almost 1,000 dark matter-rich galaxies in the Coma Cluster.

Hubble imaging picked up one of the dark galaxies. Its smattering of red stars is barely visible against the backdrop.Pieter van Dokkum & others
Hubble imaging picked up one of the dark galaxies. Its smattering of old, red stars is barely visible against the backdrop.
Pieter van Dokkum & others

Late last year, we reported on a weird find: 47 "dark" galaxies mingling with the other denizens of the Coma Cluster. Now astronomers have taken another look and discovered a whopping 854 of these faint, fluffy, and hard-to-explain objects. Their prevalence suggests they have something to say about the cluster's history.

Even ordinary galaxies contain a lot of dark matter, which makes up about 83% of the universe's mass. That's just the way our universe was built. But the dark galaxies discovered within the Coma cluster contain even more of the exotic and invisible stuff, up to 98%.

The galaxies are incredibly dim, making them difficult to pick out even with advanced instruments such as the Hubble Space Telescope. Yet despite their lack of stars, they still each span roughly the size of the Milky Way. Only dark matter could hold this diffuse collection of stars together in the collision-prone environment of a crowded galaxy cluster.

Dark galaxies in the Coma Cluster
Subaru's archival image shows newly discovered dark galaxies circled in green. Yellow circles highlight two galaxies discovered late last year with the Dragonfly Telephoto Array.

Last year Pieter van Dokkum (Yale University) and colleagues used the low-tech, high-effect Dragonfly Telephoto Array to find the original 47 dark galaxies. Now Jin Koda (Stony Brook University) and colleagues have returned to the Coma Cluster, this time poring over archival images from the powerful Subaru Telescope on Mauna Kea. The 854 dark galaxies they discovered are unique to the Coma Cluster, suggesting that the crowded cluster environment is key to stripping the stars out of these galaxies, leaving only dark matter behind.

To read more about these results, read Subaru's press release or see the paper accepted for publication in Astrophysical Journal.


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