Dark matter does not emit electromagnetic energy, light is one form of electromagnetic energy. Dark matter is so called because it does not interact with the electromagnetic force, one of the four known fundamental forces. The other three are gravity and the two forces involved in the atomic nucleus, the weak force and the strong force.
Dark matter interacts with electromagnetic energy, photons, via gravity. There are two ways that we can observe dark matter, either through gravitational lensing or observing its effects on clusters of galaxies. The orbits of galaxies within these clusters cannot be explained by the observed and inferred luminious matter, stars and gas clouds. Something else is hold them together graviationally. That something is called dark matter for lack of a better name.
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