Chronometric Particle
From Star Trek : Freedom's Wiki
Chronometric particles are particles capable of forming a temporal vortex.
In 2373, a Borg sphere used chronometric particles to open a temporal vortex which they would use to travel back in time to the Earth year 2063 and alter Human history. However, the formation of the particles was detected by the USS Enterprise-E, which allowed the starship to follow the sphere back in time and prevent damage to the timeline. (Star Trek: First Contact)
