The Silver Ghosts were a species that had the unfortunate position of being the first contact after the Qax occupation, the era of the Third Expansion. Although the Ghosts were largely benevolent explorers, concerned more with science and experimentation than conquest, they were met with unease. In part this was due to their track record of reckless experimentation with forbidden technologies, though Man's own prejudice towards unfamiliar aliens was prominent.
The Ghosts were most unique in appearance, being large floating spheres of silvery chrome.
The Ghosts came from a world devastated by a supernova early in their history; despite being abandoned to the cold of space ("the Heat Sink" as they called it), the Ghosts survived. Initially they were allies of Man, although they were never truly trusted by the humanity that had been twice dominated and nearly destroyed by aliens. The Ghosts were often associated with forbidden experiments, as they frequently toyed with the laws of nature to advance their own knowledge. Examples of this would be the Planck-Zero AI, a disastrous experiment thought destroyed in a black hole but later revived for the dark-matter soliton star test; the hyperdrive designed to cross the visible universe to explore the distant lithium-3 event and the quagma ark; the gravity-wave weapon used during the war with Man; and the altered stars that affected the stability of matter, used as fortresses during their war for survival. Despite the efforts of ambassadors such as Jack Raoul, the Ghosts eventually became an enemy of the Interim Coalition and were eventually hunted to extinction. This happened despite the fact that the Ghosts were never aggressive; indeed, the first contact between the two species saw a Silver Ghost sacrifice itself to save a stranded human astronaut. Even so, a paranoid humanity saw the Ghosts as a threat and sought to wipe them out, leaving behind only the useful pieces of their technology—even the silvery skins that the Ghosts used to protect their fragile inner bodies.
Although the Ghosts are commonly thought extinct by the time of Exultant, a faction of Man has revived the species on the remote Port Sol in the outer solar system. Although these Ghosts are completely detached from their earlier culture, they resemble the originals in many ways. The Ghosts remain tragic figures in the relentless Third Expansion and Assimilation Era, examples of humanity driven to unprecedented anger and drive to conquer. The fate of the Ghosts is revealed in one of Baxter's standalone stories. Pursued into extinction by Man, the Ghosts slip into a different universe created or chosen by the Xeelee as a gift for them, a universe of mathematical perfection that would please the Ghost mind.