Hubble's 3d map of the Dark Matter in the Universe.
Dark Matter is a form of matter that is very difficult to detect using normal methods, however it forms a large part of the Milky Way Galaxy. Even at close range, it cannot be scanned by 24th century Starfleet scanners. This is likely because Dark Matter is composed of Weakly Interacting Massive Particles, or WIMPS, which are generally not affected by the Electromagnetic Force. There are certain alien species known to be made of dark matter, such as the Photino Birds. The Totality is also composed of Dark Matter, specifically all of the Dark Matter in the Universe. (Star Trek franchise: "Star Trek: Voyager", Xeelee Sequence: "Ring")