Protomatter is a highly dangerous, unstable form of matter. Its hazardous properties led to its being demounced 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.
In 2370, protomatter was used by Professor Gideon Seyetik in an attempt to reignite the star Epsilon 119. Also in that year, the Maquis incorporated protomatter in an implosion device designed to overload the fusion drive of the Cardassian warship Bok'Nor.
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.