There are two primordial cosmic deities. A male deity personifying life, and a female deity personifying death, who is his mate. Basically yin/yang. All organic life in the universe is the spawn of the "life" god, except for the xenos which are the spawn of the "death goddess". The xenos are essentially "anti life", and their relationship with life is analogous to the relationship between matter and antimatter; they annihilate each other.