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A mysterious form of matter, detectable through its gravitational effects on visible matter. Dark matter is exceedingly common - estimates suggest that it may form as much as 80% of the universe's matter - but its nature is almost completely unknown. Dark matter is not merely ordinary matter in a cold, dark state (as for example in the case of black dwarfs or black holes), but rather seems to be of an exotic type that remains unknown. Although many possible solutions have been proposed, all of these remain, for the present, in the realm of speculation.
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