The overarching plot of the Xeelee Sequence involves an intergalactic war between humanity and the Xeelee, and a cosmic war between the Xeelee and the Photino Birds, with the latter two being alien species that originated in the early universe. The technologically advanced Xeelee primarily inhabit supermassive black holes, manipulating their event horizons to create preferable living environments, construction materials, tools, and computing devices. The Photino Birds are a dark matter-based species that live in the gravity wells of stars, who are likely not aware of baryonic life forms due to dark matter's weak interactions with normal matter. Due to the inevitable risk of their habitats being destroyed by supernovae and other consequences of stellar evolution, the Photino Birds work to halt nuclear fusion in the cores of stars, prematurely ageing them into stable white dwarfs. The resulting dwarfs provide them with suitable habitats for billions of times longer than other types of stars could, but at the expense of other forms of life on nearby planets. The Photino Birds' activities also effectively stop the formation of new black holes due to a lack of Type II supernovae, threatening the existence of the Xeelee and their cosmic projects.