Omega particles: particles capable of producing limitless energy from tiny, tiny amounts of it if held stable.
But.... if the reaction goes out of control they destroy subsapce. That's right kids, no FTL anymore.
Thus: they are hyper classified (and banned): only Captains and above are allowed to know about them. Hence a cool intro to voyager where the ship shudders out of warp, and the computer shuts down all sensors to prevent the crew (except the Captain) from seeing what's out there.
Oh, and as an aside they represent the perfect particle... thus being the 'holy grail' for the borg. But that was the crappy 'mandatory 7 of 9 B plot' and is best ignored.
later days!
Mark
'Wish I could Help you....Wish I could tell you,
That I am real, I'm not something you invented,
That I'm not everything you want me to be.'
'And I am...Ageless. And I am....Invincible.'