The Uxali are a humanoid species native to the Delta Quadrant planet Uxal. Their planet was a peaceful one, on the path to advancement when an Earth probe, Friendship 1 from across the galaxy entered their atmosphere. It told them about the Humans, about how to communicate, and about how to build warp drive technology.
Prior to this incident, the Uxali had no idea that antimatter existed. One of the early experiments with it following resulted in a massive antimatter explosion, polluting the once M-class atmosphere with poisonous gas and radiation. Many Uxali were killed, and the few that survived were forced to live in underground caverns that partially protected them from the radiation. (VOY: "Friendship One")