Most of the universe’s heft, oddly enough, could come in the form of particles billions of times lighter than the electron — a featherweight itself, as particles go. Streaming through the cosmos in thick hordes, these wispy “axion” particles could deliver a collective wallop as the missing dark matter that appears to outweigh all visible matter 6-to-1.

For decades, physicists have searched for the axion’s chief rival: a sluggish and far heavier dark matter candidate known as a WIMP (for “weakly interacting massive particle”). But WIMP experiments remain empty-handed as researchers approach the edges of their search field, while the hunt for the axion is only beginning.

“Dark matter could still be WIMPs, but every day it looks a little bit less likely,” said Ben Safdi, a physicist at the University of Michigan who specializes in dark matter. The axion “is kind of the best dark matter candidate that we have at the moment,” he said, given that others have failed to turn up in experiments.

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