The omega molecule is an extraordinarily powerful particle whose instability can destroy subspace over vast distances, so dangerous that Starfleet orders captains to destroy it on sight.
A single omega molecule holds as much energy as a warp core, but an unstable one can rupture subspace across a wide region and make warp travel there impossible. Because of this, Starfleet created the Omega Directive, which overrides even the Prime Directive and commands captains to eliminate any omega molecule they find.
In 2374 Voyager detected omega molecules created by an alien scientist, and Captain Janeway set out to destroy them. Seven of Nine revealed that the Borg knew the particle as Particle 010 and regarded it as perfection. Before the molecules were destroyed she briefly saw them stabilize, an experience that moved her deeply.