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 percent 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
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