Rare particles detected in what appears to be a nebula, which Janeway hopes to collect to boost Voyager's antimatter reserves early in the journey home.
Omicron particles appear in the Voyager episode The Cloud. With the ship short of energy and replicators rationed, sensors find an unusually high concentration of the particles in a nearby nebula, and Janeway orders Voyager in to gather them.
The ship forces its way through an energy barrier and is covered in an unknown substance that drains its power, so that it leaves with less than it had. The Doctor then recognises the debris as organic.
The nebula is a living creature and Voyager has wounded it. The crew return to repair the damage with a nucleonic beam acting as a suture, and the hoped-for supply of omicron particles is abandoned.