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Astronomers think that galaxies cannot form without the gravitational pull of dark matter. So a trail of galaxies free of this mysterious material, with no obvious cause, would be a remarkable find. In a paper published in Nature on 18 May1, astronomers say they might have observed such a system — a line of 11 galaxies that don’t contain any dark matter, which could all have been created in the same ancient collision. But many of their peers are unconvinced that the claim is much more than a hypothesis.