In late June 2023, on a morning train commuting to work in New York City, David Hendel was scrolling through a recent astronomy paper on his phone when a half-page image caught his eye.
The image, captured a year earlier by the Hubble Space Telescope, showed UGC 9050-Dw1, a dwarf galaxy some 115 million light-years from Earth. It appeared as little more than a faint, diffuse smudge of bluish-white light that astronomers call an ultra-diffuse galaxy — a type of galaxy so sparsely populated with stars that most of its mass is thought to be dark matter, the invisible substance that makes up more than 80% of the matter in the universe but has yet to be directly detected.
What made Hendel pause was not the galaxy itself but a thin, fragile ribbon of stars trailing off from it, so dim it nearly disappeared among the other smudges of light crowding the frame. Hendel, a former astronomer who had studied thousands of similar structures, immediately recognized the subtle interplay o...
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