Jakmanite

From Memory Alpha, the free Star Trek reference

Jakmanite is a radioactive element that isn't normally picked up by the sensors of a Galaxy-class starship.

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

Jakmanite has a half-life of about fifteen seconds and was thereby ruled out as having caused the malfunctions aboard the USS Enterprise-D at the end of 2366, as there wouldn't have been enough time to spread the substance all over the ship. (TNG: "Hollow Pursuits")