Xeelee Sequence

The Xeelee Sequence (/ˈziːliː/; ZEE-lee) is a series of hard science fiction novels, novellas, and short stories written by British author Stephen Baxter. The series spans billions of years of fictional history, exploring humanity's future expansion into the universe, its intergalactic war with the Xeelee, a supremely advanced Kardashev Type V alien civilization, and the Xeelee's own conflict with dark matter entities known as Photino Birds. The Xeelee are described as symbiotic entities composed of spacetime defects, Bose–Einstein condensates, and baryonic matter.