Photino Birds are an 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. Unaffected by the physical heat of the star, the creatures simply orbit its gravity well.
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. 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 the ability to freely and non-paradoxically maneuver through time: an advantage the Xeelee could not overcome. The creatures do not die naturally, not even by the end of the universe (they simply time travel back to the beginning); they can even use time travel to replicate themselves and travel instantly to any point in space. 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 adverse gravity field.
Seeing that the war wasn't going well for them, the Xeelee built a vast structure called Bolder's Ring (Or the Great Attractor, or just the Ring). The Photino Birds ultimately won the war with the Xeelee, and the series ends with them in triumphant control of a cold, dark universe. Before their ultimate triumph, the Xeelee, Humanity, and perhaps some other unnamed species escaped through the Ring before it was destroyed by the Birds during a Qax-Bird battle. During the battle, the last remnants of humanity escaped.