Coal-fired power plants are a big source of particle pollution.

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The appeal hearing was told it was "just as likely" that the particle came from "some extraneous source as it was that it came from a gun fired".

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"There's a huge synergy there, in astronomers trying to find the influence of dark matter by mapping stars and galaxies and large structures in the universe, and particle physicists trying to discover the source of that influence of dark matter through subatomic particles here on Earth, " said Jason Kalirai, deputy project scientist for the telescope at the Space Telescope Science Institute.

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