The Xeelee Sequence (; ) 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 intergalactic war with an enigmatic and supremely powerful Kardashev Type IV alien civilization called the Xeelee (eldritch symbiotes composed of spacetime defects, Bose-Einstein condensates, and baryonic matter), and the Xeelee's own cosmos-spanning war with dark matter entities called Photino Birds. The series features many other species and civilizations that play a prominent role, including the Squeem (a species of group-mind aquatics), the Qax (beings whose biology is based on the complex interactions of convection cells), and the Silver Ghosts (colonies of symbiotic organisms encased in reflective skins). Several stories in the Sequence also deal with humans and posthumans living in extreme conditions, such as at the heart of a neutron star ("Flux"), in a separate universe with considerably stronger gravity ("Raft"), and within eusocial hive societies ("Coalescent").