Dark matter: all mass and no substance, this theoretical type of matter is used by physicists to explain why the universe acts as if it has five times more mass than we can see–basically if galaxies only had the mass that is represented by the visible matter in them, the strain that their rate of spin puts on them would cause them to fly apart. And this strange, invisible substance that doesn’t seem to interact at all with ordinary matter should be distributed as evenly as the rest of the matter in the universe… except that astronomers have recently found a galaxy that contains no dark matter at all, demonstrating that its theoretical presence isn’t as ubiquitous as we thought.
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