The Xeelee Sequence (/ˈziːliː/; ZEE-lee) is a series of hard science fiction space opera novels, novellas, and short stories written by British science fiction author Stephen Baxter. The series spans billions of years of fictional history, centering on humanity's future expansion into the universe, its cosmos-spanning war with an enigmatic Type IV alien civilization called the Xeelee, and the Xeelee's own war with dark matter entities called Photino Birds. The series features many other species and civilizations that play a prominent role, including the Squeem, the Qax, and the Silver Ghosts. Several stories in the Sequence also deal with humans and posthumans living in extreme conditions, such as the heart of a neutron star (Flux), a separate universe with considerably stronger gravity (R