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In 2016, astronomers led by Pieter van Dokkum of Yale University published a bombshell paper claiming the discovery of a galaxy so dim, yet so broad and heavy, that it must be almost entirely invisible. They estimated that the galaxy, dubbed Dragonfly 44, is 99.99% dark matter.

A heated debate ensued about Dragonfly 44’s properties that remains unresolved. Meanwhile, more than 1,000 similarly big but faint galaxies have turned up.

Dragonfly 44 and its ilk are known as ultra-diffuse galaxies (UDGs). While they can be as large as the largest ordinary galaxies, UDGs are exceptionally dim — so dim that, in telescope surveys of the sky, “it’s a task to filter out the noise without accidentally filtering out these galaxies,” said Paul Bennet, an astronomer at the Space Telescope Science Institute in Baltimore. The bright star-forming gas that’s abundant in other galaxies seems to have vanished in UDGs, leaving only a skeleton of elderly stars.

Their existence has caused a stir in galactic evolutionary theory, which failed to predict them. “They didn’t turn up in simulations,” van Dokkum said. “You have to do something special to make a galaxy that big and faint.”

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