One of the mysteries of the English language finally explained.
The amount by which a measured or calculated mass falls short of an expected or inferred mass; (Astronomy) the discrepancy between the mass of a galactic cluster calculated from the velocities of its component galaxies and the mass of the visible (luminous) stars and gas within it, usually attributed to the presence of dark matter; (also) the difference between the mass there must be in the universe if it is closed and the total mass accounted for by direct observation.
Late 19th century; earliest use found in American Naturalist. From missing + mass.
missing mass/ˌmɪsɪŋ ˈmas/
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