The Night Aliens were a species that were indigenous to a region of space in the Delta Quadrant, dubbed by the crew of the USS Voyager as "the Void." They claimed to have inhabited the region for millions of years , undisturbed until the arrival of the Malon.
Their physiology had evolved to survive in the complete darkness of the Void. Among other things, they were extremely photosensitive. Technologically, they were relatively advanced, possessing starships that used dampening fields to drain energy from encroaching ships.
The Night Aliens had been suffering from toxic effects of theta radiation exposure from antimatter waste, that were being dumped into their region of space by the Malon. They were later encountered by the crew of the Voyager in 2375, who assisted the Night Aliens in stopping the Malon from any further pollution and destroying spatial vortex that led to that region. (VOY: "Night")