Astronomers have studied a trio of ultra-diffuse galaxies located 65 million light-years from Earth that contain almost no dark matter. Star movements indicate these objects are made almost entirely of ordinary matter, which researchers suggest was stripped away during an ancient galactic collision while the dark matter passed straight through. If correct, this mechanism explains how galaxies can form without their invisible gravitational scaffolding.
Finding galaxies devoid of dark matter provides a useful constraint for theoretical models that try to explain what the substance actually is. At the same time, it puts pressure on alternative theories that attempt to replace dark matter entirely with modified gravity. We are still a long way from catching a dark matter particle directly, but watching how ordinary matter behaves when the invisible stuff goes missing remains one of the cleaner tests we have.
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