the.com/dark matter

means A form of matter that neither emits nor absorbs light, detected only through its gravitational pull on visible stars, gas, and galaxies.

from 'Matter' traces back to Latin materia, 'substance' or 'timber' — the stuff things are built from. 'Dark' is just the plain old English word for lightless. The phrase itself is a 20th-century coinage from astronomy: observers noticed galaxies rotating far too fast to be held together by the visible mass alone, so physicists named the missing, unseen mass that must be supplying the extra gravity — dark not because it is black, but because it gives off no light at all, making it invisible to every telescope ever built.