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.