The nucleogenic cloud being was a nebula-like lifeform encountered in the Delta Quadrant. The lifeform measured seven AUs in diameter and was composed of hydrogen, helium, and hydroxyl radicals, with some dust nodules present also. The being used multi-polar charges for defense.
While looking for omicron particles on stardate 48546.2, in 2371, the USS Voyager accidentally harmed a member of this species by entering its body, not knowing it was a living entity. The crew of Voyager performed a suture with nucleonic beams to assist the being in healing itself. (VOY: "The Cloud")