Dark Matter is an RPG game world published by TSR (now Wizards of the Coast). It is a modern day setting dealing with conspiracy lore, the paranormal, the occult, UFOs and extraterrestrials, etc.

From the back cover:

Think back to every story of alien visitors, psychic powers, occult lore, unexplained miracles, strange cults, and secret societies that you've ever heard... then imagine if they were all true, and your job was to investigate them.

This new campaign setting for the Alternity science fiction roleplaying game rips away the comforting veils of ignorance, exposing the world's corruption and rot for all to see. Devious organizations scheme for world domination, otherworldly forces infiltrate our power structures, and creatures from our nightmares lurk in the shadows. Working for a clandestine organization called the Hoffmann Institute, heroes explore hidden mysteries while eluding forces--both human and alien--that scheme to control the truth.