The nucleogenic cloud being was a nebula-like lifeform native to the Delta Quadrant. The lifeform measured seven AUs in diameter and was composed of hydrogen, helium, and hydroxyl radicals. The being used multi-polar charges for defense.
While looking for omicron particles in 2371, the USS Voyager harmed a member of this species by entering its body, not knowing that it was a living entity. It eventually was able to heal itself with assistance of Voyager, which performed a "suture" with power flow. (VOY: "The Cloud")