Definitions for dark matter
Random House Webster's College Dictionary
a hypothetical form of matter invisible to electromagnetic radiation, postulated to account for gravitational forces observed in the universe.
Origin of dark matter:
(cosmology) a hypothetical form of matter that is believed to make up 90 percent of the universe; it is invisible (does not absorb or emit light) and does not collide with atomic particles but exerts gravitational force
Particles of matter that cannot be detected by their radiation but whose presence is inferred from gravitational effects.