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")
These "alien lifeforms" were initially described in the revised final draft of the script as "Humanoid, iridescent, strange. Its eyes all black, all pupil. The creature reacts with pain at the light. This is a lifeform that has evolved in complete darkness. (We will call this creature a "Night Alien")"