Z-particle

The z-particle is a type of subatomic particle. In particular, the z-particle has an electrical charge of zero and is its own antiparticle.

History and specifics
Sensors on the Federation starship USS Enterprise (NCC-1701-D) showed that z-particles were emitted when Armus used his powers Enterprise was at planet Vagra II in the year 2364. The crew were able to use sensor scans of z-particle emissions as an indicator of Armus's power level. 

Geordi La Forge scanned for z-particles, among other things, when he was investigating the unexplained disappearance of former crewmates of the USS Victory on Tarchannen III. None of these particles were detected.