A galaxy is much more than a radiant agglomeration of stars. To modern astrophysicists, galaxies are more notable for their dark sides: their hidden material that is only “seen” by its gravitational pull upon the shiny stuff it seems to vastly outweigh. So-called dark matter is as much a defining feature of galaxies as stars and gas, and is thought to provide the gravitational seeds from which galaxies assemble and grow.

A galaxy without dark matter—or without some bizarre effect of gravity that would mimic dark-matter behavior—would be a very weird thing indeed. Finding such a thing would be like finding smoke but no fire, effect without cause. Yet that is what Yale University astronomer Pieter van Dokkum and his colleagues have just found, they report in a study published Wednesday in Nature.

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