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M31, the Andromeda galaxy, is the closest galaxy like our own

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Now, NASA's Fermi mission reports a strange gamma-ray signal at its center

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This could be a signal of mysterious dark matter,

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or the collective glow of many spinning neutron stars, called pulsars.

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Astronomers don't yet know.

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Fermi has found a similar unexplained signal at the center of our own galaxy