Selgninaem sulfide
Selgninaem sulfide, or just Selgninaem, was a substance that wasn't normally picked up by the sensors of a  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 could exist in a nitrogen-oxygen atmosphere and could alter the molecular structure of glass.

Selgninaem was highly toxic and was thereby ruled out as having caused the malfunctions aboard the  in 2366, as the whole crew would have died of poisoning. ()

ja:セルグネンネイウム
Chemical compounds