Protomatter is an unstable substance whose use in the Genesis Device by David Marcus caused the Genesis planet to destroy itself.
Most scientists regard protomatter as too dangerous to use. David Marcus nonetheless incorporated it into the Genesis matrix to overcome problems he could not otherwise solve, a shortcut he admitted only when the consequences became clear.
The protomatter made the newly formed Genesis planet unstable, ageing it at a tremendous rate until it tore itself apart. It also drove the rapid regeneration and ageing of Spock's body on the planet's surface. The flaw made Genesis useless as a peaceful tool and dangerous as a weapon.