What is dark matter?

A、Darkmatterissomethingdarkandhardtorecognize.
B、Darkmatterissomethingsecretandcan'tbeexposed.
C、Darkmatteriswhatwecallmysteriousunknownworld.
D、Darkmattermakesupmostofouruniverse,containinglotsofgalaxiesandstars.