Selgninaem sulfide, or just Selgninaem, was a substance that wasn't normally picked up by the sensors of a Galaxy-class starship. This substance was processed in the Rigellian colonies for use as a catalyst in industrial biochemical systems.
Furthermore, along with invidium, saltzgadum, lucovexitrin, and jakmanite, it was one of only five undetectable substances that can exist in a nitrogen-oxygen atmosphere and can alter the molecular structure of glass.
Selgninaem is highly toxic and was thereby ruled out as having caused the malfunctions aboard the USS Enterprise-D at the end of 2366, as the whole crew would have died of poisoning. (TNG: "Hollow Pursuits")