Photino Bird

The photino birds are an ancient and extremely powerful species of dark matter entities from Steven Baxter's Xeelee Sequence. They are the rivals of the Xeelee and, while they appear only rarely, they are the overarching antagonists of the setting.

Description
Photino birds are composed entirely of dark matter. Consequently, they interact with normal matter only gravitationally. They are thus invisible, intangible, and effectively invulnerable; only extreme gravitational fields can harm them.

Anatomy
Photino birds bear almost no resemblance to actual birds. They are instead roughly disk-shaped, and measure about 50 meters (160 feet) across. They have considerable body mass, albeit of unknown value, and can use it as a weapon by overlapping themselves en masse across a baryonic target, increasing the amount of mass in the target volume until the increased gravity crushes it.

Diet
Photino birds leach thermal energy from the cores of stars.

Reproduction & Life Cycle
Photino birds reproduce by budding. The offspring is smaller than its parent but inherits all of its memories. Photino birds are effectively immortal.

History
Photino birds emerged in the earliest ages of the universe, not long after the Xeelee themselves. While they were composed of dark matter, they needed baryonic matter to live - they fed on the heat from hot baryonic matter and relied on dense baryonic matter to aid in their reproduction. They thus lived mostly in the cores of stars. The supernovae of early stars proved an unacceptable disruption to their lifestyle, so they set about altering the universe's stars, making them safer for themselves by vastly accelerating their transformation into white dwarfs. A universe of white dwarfs, however, would form no new stars or black holes and would be utterly unlivable for almost all baryonic life. Thus, the Xeelee intervened, making war against the photino birds in an attempt to stop their project.

The photino birds had an insurmountable advantage, since dark matter was vastly more abundant than baryonic matter. It soon became apparent to the Xeelee that the photino birds would inevitably win. The Xeelee began constructing Bolder's Ring, an escape hatch to other universes that would occupy their efforts for the next ten billion years. Around the same time that the Earth began to form, about five billion years into the Ring's construction, the birds noticed the Ring. Fearing (falsely) that it was a weapon or that the Xeelee would use another universe as a staging ground to take back their own, the photino birds began to attack it.

The photino birds' attacks on the Ring and the stars of the universe continued for billions of years. At some point they were discovered inside Sol by humans, by which time they were thought to have infested every star in the universe. Humans and photino birds did not otherwise interact, and the photino birds do not appear to have ever learned that humans even existed. For their part, humans were only peripherally aware of the photino birds' star-aging project or their war with the Xeelee.

By 5,000,000 AD, the photino birds had all but finished with the universe, and their conflict with the Xeelee came to a head in a cataclysmic final battle at Bolder's Ring. Xeelee, humans, and other species fled through the Ring, while a force of Qax held off the photino birds long enough for a mass evacuation to take place before the Ring was destroyed.

Following the battle at Bolder's Ring, the photino birds completely dominated the universe, and all life that had been unable to escape died with the stars that had sustained them. The only known surviving pocket of baryonic life was Old Earth, which had been artificially heated and placed in a "time sink" so that time passed far slower on the surface than in the outside universe. Billions of years after their victory, some photino birds adopted a symbiotic relationship with the humans living there.