dark matter

WordReference Random House Unabridged Dictionary of American English © 2025

dark′ mat′ter, 

1. Physics a hypothetical form of matter invisible to electromagnetic radiation, postulated to account for gravitational forces observed in the universe.

• 1985–90

Collins Concise English Dictionary © HarperCollins Publishers::

dark matter noun

1. matter known to make up perhaps 90% of the mass of the universe, but not detectable by its absorption or emission of electromagnetic radiation