Saltzgadum

From Memory Alpha, the free Star Trek reference

Saltzgadum is a substance that isn't normally picked up by the sensors of a Galaxy-class starship.

Furthermore, along with invidium, lucovexitrin, selgninaem and jakmanite, it is one of only five undetectable substances that can exist in a nitrogen-oxygen atmosphere and can alter the molecular structure of glass.

Saltzgadum hadn't been used aboard Federation starships for decades, so it was ruled out as having caused the malfunctions aboard the USS Enterprise-D at the end of 2366. (TNG: "Hollow Pursuits")