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(proposed light, electrically neutral elementary particle)

An axion is a proposed elementary particle, which would explain some experimental results in particle theory, and could comprise cold dark matter.

The axion was theorized in 1977 to fill out the standard model of particle physics to resolve the strong CP problem, i.e., explaining why the strong force never violates CP symmetry, a symmetry which is never seen violated, but no reason is known why. If axions exist and have mass, they are a candidate for cold dark matter.

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