Because there were no stars or other systems in the area that produced light, this race was fully adapted to living in total darkness. Technologically, they were relatively advanced and used a dampening field to drain energy from encroaching ships.
The Night Aliens had been infected by toxic chemicals, and theta radiation from antimatter waste, that were being dumped into their region of space by the Malon. They were later encountered by the crew of the USS Voyager in 2375, who assisted the Night aliens in stopping the Malon from any further pollution. (VOY: "Night")