Neutrinos are subatomic particles which can be used for a variety of purposes. Federation starships have neutrino detectors included in there sensor arrays, and the presence of elevated neutrino levels could indicate the presence of a ship, possibly cloaked. The antiparticle of the neutrino is the antineutrino.
Natural phenomena have also been known to produce neutrinos, such as the Bajoran wormhole. It was the detection of elevated neutrino levels that allowed a team from Deep Space 9 to discover the wormhole in the year 2369. Neutrinos were also emitted by the wormhole before it opened, allowing the DS9 crew advance warning of a vessel coming through. (DS9 episode & novelization: Emissary)
Following the disappearance of a Danube-class runabout carrying several Starfleet admirals near the Bajoran wormhole in 2370, Lieutenant Jadzia Dax noted that the neutrino output of the wormhole had become erratic. It was later discovered that it was due to interference by the Cardassians and the "Othersiders". (TNG - DS9: The Landmark Crossover comic: "Prophets and Losses")