The Photino Birds are the primary antagonists of Stephen Baxter's Xeelee Sequence series of science fiction novels. They are a species composed of dark matter that have colonized all of the stars in the universe, and are in the process of transforming the stars into white dwarves so they will not become supernovae or black holes. This will eventually lead to the extinction of all baryonic (non-dark matter) life in the universe.
Being made of dark matter, the Photino birds are invisible and intangible, and immune to most forms of damage. Heat, electromagnetism, kinetic energy and nuclear forces are all useless against them. The only thing that can harm them is gravity, such as the intense gravity of a black hole or a Xeelee starbreaker beam. The Photino Birds live and reproduce inside stars, and they make up the majority of the mass of the universe. They have the ability to shut down the nuclear fusion inside of stars, slowly transforming them into white dwarves.
They can also manipulate gravity, such as accelerating entire galaxies to near lightspeed to use as weapons. They can travel back and forward through time and use time travel to create copies of themselves, making up for any that are killed. They are also immortal and do not age or die naturally.
Once the Xeelee realized they could not defeat the Photino Birds, they attempted to build a device known as the Ring to open a portal to another universe, however the Photino Birds attempted to destroy it to prevent them from escaping. They were ultimately unsuccessful, and the Xeelee, as well as humanity, escaped the dying universe.