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The humble neutron could be hiding a deep, dark secret. For 20 years, two experiments that measure the lifetime of a neutron have been at odds. Now it seems that disconnect may be the result of neutrons occasionally decaying into particles of dark matter, the stuff that is thought to make up most of the unseen mass of the universe.

“What we thought were our most likely ideas for what dark matter might be, none of those have worked,” says at Fermilab near Batavia, Illinois, who wasn’t involved in the research. “It seems well worthwhile to consider…