Protomatter is a highly dangerous, unstable form of matter.
Its hazardous properties led to its being denounced by most 23rd century scientists, but this did not stop David Marcus from secretly using it in his Project Genesis matrix. This doomed the project to failure, however, because the planet created by the matrix shared its instability. (Star Trek III: The Search for Spock)
In 2370, protomatter was used by Professor Gideon Seyetik in an attempt to reignite the star Epsilon 119. (DS9: "Second Sight")
Also in that year, the Maquis incorporated protomatter in an implosion device designed to overload the fusion drive of the Cardassian warship Bok'Nor. (DS9: "The Maquis, Part I")
In 2373, a changeling infiltrator impersonating Deep Space 9's Dr. Bashir used protomatter, along with trilithium and tekasite, in an attempt to destroy the Bajoran sun. (DS9: "By Inferno's Light")
In 2374 the USS Voyager encountered a nebula that contained high concentrations of protomatter. Neelix, who had worked with protomatter in the past was killed while protomater was transported into a canister he was holding. Luckily, he was brought back to life using Borg technology. (VOY: "Mortal Coil")