Photino Birds orbiting a star.
Photino Birds is the informal name given to a species of dark matter creatures which are colonizing all the stars of the universe, feeding off the energy in their cores. Being made of dark matter, they are unaffected by the physical heat of the star, simply orbiting its gravity well. Photino Birds gain sustenance by staying within stable, non-extreme wells of gravity. While a main sequence star provides such gravity, it also runs the risk of destabilizing into a supernova which in turn collapses into a neutron star and ultimately into a black hole; black hole gravity is too extreme for their purposes. Therefore, the Photino Birds engage in a campaign to prevent supernovae by prematurely stalling the fusion reaction of every star in the universe. This will cause these stars to stop emitting energy and instead cool into white dwarves: still massive enough to exert gravity for the Photino Birds but completely stable.
These creatures are effectively terraforming the entire universe, an amazing feat for any sentient species. Unfortunately, this makes the universe ultimately inhospitable to baryonic lifeforms, such as Humans, Qax, Xeelee, and so on. Baryonic life needs the energy emitted by main sequence stars to maintain life on their worlds. Furthermore, the Xeelee rely on black holes to power their own technology; a universe full of white dwarves will permanently prevent this. Because of this, the Xeelee opened a vast, universe-wide war with them in a desperate effort to preserve baryonic life from extinction.
However, Photino Birds carry with them one unassailable advantage the Xeelee could not overcome; they are not bound by time. They can freely exist in and travel to any point in time. The creatures do not age nor die naturally, not even by the end of the universe; they just travel back to the beginning of time, including to points so soon after the Big Bang that baryonic matter (including the Xeelee) could not yet exist. As creatures of dark matter, most forms of matter and energy simply pass through them, yet they can manipulate the same matter and energy almost with impunity; propelling entire galaxies to near the speed of light is possible for them. The only thing that can adversely affect and ultimately kill them is an extreme gravity field, like that of a black hole. Yet, due to their not being bound by time, such losses can be quickly replaced by using time travel to replicate themselves (without being subject to the paradox of two temporal selves co-existing) or to instantly travel to any point in space.
Once the Xeelee realized the Photino Birds could outmaneuver them through time and therefore were impossible to defeat, the Xeelee built a vast structure called Bolder's Ring (Or the Great Attractor, or just the Ring) designed to create a rift into another dimension where baryonic life could escape. As the rift opened, the Xeelee passed through, then various other baryonic life. The last to leave were the last remnants of humanity, who tricked the Qax into attacking the Photino Birds as a distraction. Soon after they passed through, the Birds destroyed the Ring in the course of the battle with the Qax. The universe was abandoned, and the series ends with the Birds in triumphant control of a cold, still universe until, billions of years later, proton decay caused the matter within to decompose.