Dark matter
From Memory Alpha, the free Star Trek reference.
Dark matter is a form of matter that is undetectable using conventional sensors. It forms the basis of several types of spatial anomalies, such as dark matter nebulae (TNG: "In Theory").
Although previously thought to be impossible, dark matter lifeforms were discovered by the Delta Flyer in a class T cluster in 2376. (VOY: "Good Shepherd")
