Protomatter was a highly dangerous, unstable form of matter.
Its hazardous properties led to its denunciation by most 23rd century scientists, but this did not stop David Marcus from secretly using it while trying to solve certain fundamental problems inherent in the Project Genesis matrix. By doing so, he inadvertently doomed the project to failure as it caused the planet formed by the matrix to inherit the unstable properties of the protomatter. (Star Trek III: The Search for Spock)
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 protomatter was transported into a canister he was holding. Seven of Nine subsequently brought him back to life with her Borg nanoprobes. (VOY: "Mortal Coil")