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The Xeelee // are a fictional hyperadvanced species from Stephen Baxter's Xeelee Sequence. They were first remotely mentioned in the 1994 novel Timelike Infinity and were later central actors of several novels and a substantial number of short stories. Within the Xeelee Sequence, they are considered to be the most advanced of all Baryonic life-forms and possess technology and abilities far beyond any other spacefaring civilization. They have demonstrated the ability to routinely construct Closed Timelike Curves and have exploited time travel to engineer their own evolution and history as far back as 13.5 billion years ago (within two hundred million years of the Big Bang). Their abilities in the series far exceed that of a Type III Civilization on the Kardashev scale.
Although the Xeelee seldom directly appear or take action in the stories their impact on the development of most civilizations and the universe as a whole is profound. The Xeelee frequently leave behind derelict ships, tools, and other materials giving the appearance of thoughtlessness or reckless abandon at what a junior species might do with it. Most junior species see no benefit in developing a technology on their own when they can simply steal it from the Xeelee debris. This has a profound influence on the various galactic economies and technological development of junior species.
In the Xeelee sequence life and intelligence are ubiquitous throughout the universe. The universe has a fundamental tendency toward complexity and autocatalystic systems are easily created. This is not limited to particle matter, but also applies to more exotic structures such as dark matter, quarks, and flaws in space-time itself. Thus, life emerged when the universe was extremely young, to within Planck Time of the Big Bang, and there were intelligent civilizations that witnessed, and indeed consciously influenced, major events in the early universe such as inflationary expansion, symmetry breaking, and various phase changes. At the close of each of these epochs, as the universe took on a fundamentally different characteristic, there was a corresponding mass extinction, but small enclaves of the intelligences would survive in one manner or another, witnessing the universe becoming progressively colder and slower-reacting.
The novel Exultant describes the progenitor of the Xeelee, referred to as the proto-Xeelee, as one of the few survivors of the last phase change (when the universe had cooled enough for atoms to form). The proto-Xeelee established symbiotic relationships with other survivors as well as autocatalystic reactions in the new baryonic matter, thus emerged what would later be called the Xeelee. This potentially dates the Xeelee to when the universe was only a few hundred thousand years old, but with direct knowledge and observations that reach back to within minutes of the Big Bang.
The Xeelee used primordial black holes as a habitat, construction tool, and computing devices because, at the time they emerged, the event horizon was one of the few thing that survived from earlier epochs that they understood. More so, heavier atoms were exceedingly rare because stars had not yet had the time to create them through nuclear fusion. As the primordial black holes evaporated over time the Xeelee eventually migrated to using the supermassive black holes around which Galaxies were forming. Although the galactic black holes were trillions of times more massive than their ancestral homes, the Xeelee proved to be quite adept at using the complex physics around the event horizon for their own purposes. Over the billions of years since their migration, the Xeelee have become a ubiquitous part of the complex ecology of the super massive black holes along with an unnamed number of other species that are also veterans of the same very early eras of the universe.
There is no explicit physical description of the Xeelee throughout the series. In the novel Exultant it is alluded that the over their exceedingly long history, the Xeelee have combined themselves with their technology and, as such, may not have a distinct individual presence that Humans would be familiar with.
War with the Photino Birds 
The Photino Birds are a species of dark matter that prefer a habitat in the gravity well of stars. In general, the photino birds are probably unaware of almost all other forms of life due to the fact that dark matter interacts extremely weakly with baryonic matter (normal matter that is composed of atoms). The Photino Birds recognize a risk to their habitats from supernovas and other consequences of normal stellar evolution and have set themselves to ceasing nuclear fusion in the cores of stars, thereby prematurely aging them into white dwarves. The resulting dwarves will have the same gravity well but be stable for billions of times longer than their progenitor. Because white dwarfs are extremely cold and dim compared to typical stars, they would not support life on nearby planets. The long project of the Photino Birds will ultimately make the universe hostile to baryonic life, and they have spread across the universe. Further, the activities of the Photino Birds will effectively halt formation of new black holes due to a lack of Type II (core-collapse) supernova. This effectively puts the Xeelee and the Photino Birds at irreconcilable odds because each species is fundamentally seeking to threaten the ideal habitat of the other.
The Xeelee became aware of the Photino Birds approximately 13.5 billion years ago. Seemingly as a unified species, the Xeelee have concerned themselves with defeating the Photino Birds for almost the entire history of the universe. Despite their state of high technological advancement, the Xeelee are ultimately unsuccessful.
War with the Humans 
While recovering from a brutal occupation by the Qax that enforced stagnation and flirted with extinction, Humanity adopted an extremely xenophobic imperative that aimed to ensure the future of the species (known as the Druz Doctrines after its founder, Hama Druz). Eventually Humanity again began to expand into the galaxy, wiping out an uncounted other species during their advance. Eventually Humans became the second most advanced and widespread race in the galaxy, second only to the Xeelee. Humans had no knowledge of the Photino-Xeelee war or even what was at stake, making the Xeelee actions interpreted as being sinister. Building the Ring had the appearance of being destructive on a galactic scale because its larger purpose was not understood until the very far future, and many smaller Xeelee projects, such as changing the orbit of Callisto to preserve the bacteria living in the ice, lacked an obvious explanation. Though there were several orders of magnitude fewer Xeelee than humans known to be in the Galaxy, their high degree of technical advancement made them a formidable enemy.
Through a bitter war of attrition, Humans eventually contained the Xeelee within the Galactic Core where the battle lines stagnated for at least three thousand years. Both humans and the Xeelee gained strategic intelligence through time travel by constructing Closed Timelike Curves, further contributing to the stagnation. Human casualties were estimated to exceed more than 10 billion per year and more than 30 trillion over the course of the assault. The imperative commitment to total war also stagnated human physical and intellectual evolution. The Druz Doctrines did not encourage exploration or research and there was little new development. Further, very little effort was directed toward projects that did not have an immediate and conspicuous benefit to maintaining the massive logistics tail necessary to continue the assault.
Eventually the Humans developed a means of hiding their actions from the Xeelee by making movable pocket universes that had no causal relationship to the universe as a whole, thereby eliminating any advantage of moving information into the past with time travel. Humans further developed a time-travel based computer that allowed humans arbitrary computer power to outthink the Xeelee by analyzing tactical problems (giving the pilot tactical information to defeat Xeelee maneuvers) and a dual black-hole cannon that would fire two singularities to merge at a known distance away, releasing enormous gravity waves in the process. Knowing that the ecology around the event horizon and the creatures that live within it would be damaged or destroyed if humans continued to batter the event horizon of the supermassive black hole, the Xeelee withdrew from the Milky Way within minutes after the humans scored the first successful hit.
No longer having the Xeelee as a unifying threat, humanity fragmented into multiple governments over the next several hundred years. The doctrinal stagnation collapsed and Humanity advanced technologically over the next 100,000 years. Humanity then unified again to begin directly attacking Xeelee concentration in the greater supercluster of galaxies across the local group. Humans proved only a slight annoyance or distraction to the Xeelee.
Interactions with other species 
As far as is known and aside from the Photino Birds, the Xeelee were seldom warlike and usually treated the junior species with disinterest or abandon. There are very few reported instances of them initiating interaction with another species. They will, however, defend their habitats through strength of arms and respond to attacks, as was the case in the war with the Humans. It is noteworthy that during the war with the Humans, the Xeelee rarely opened an offensive or attacked far behind the front despite having an enormous ability to do so. In the end, the humans' total commitment to war amounted to a minor irritation to the Xeelee and their response was minimal at best.
The Xeelee have, however, demonstrated several instances of compassion en mass and even charity toward other species, particularly when it involved escaping the Photino Birds' destruction of the habitable universe. In the conclusion to novel Ring, set 5 million years in the future, they did not interfere with the humans escaping to another universe despite hundreds of thousands of years of humanity's war on the Xeelee (some interpretations are that they even provided an armed escort for the humans in the final leg of the voyage). Even though they were thought to have been driven to extinction by the humans, the Xeelee constructed a universe for the Silver Ghosts that was appealing to their mathematical nature. Before their own retreat from the universe, the Xeelee left ships as 'life boats' at various locations so that other species could use them to escape the Photino Birds through the Ring.
The Anti-Xeelee is a universe spanning quantum-wave consciousness constructed by the Xeelee to manage their projects. It was first briefly described in the conclusion of Timelike Infinity when it was encountered by Michael Poole who had been pushed several million years into the future. It was later developed somewhat more in the short story Vacuum Diagrams. Its name is derived from the particle/anti-particle naming convention to convey its paired and complementary existence with the Xeelee; it does not describe an opposition to the Xeelee. It further implied that the Anti-Xeelee, is traveling backwards in time like some anti-particles are thought to do.
Main article: Xeelee technology
The Xeelee were masters of time and space. Their starships, known as Nightfighters, are crafted from a strange form of matter that seems to violate the Pauli exclusion principle, which states that no two electrons of the same atom can be in the same state at the same time. The material is sharp, hard, and transparent. Due to its unique space-time construction, it has strange gravitational effects.
The Xeelee's greatest achievement—possibly the greatest structure of Baryonic matter ever constructed—is the Ring, a gigantic loop of cosmic string, millions of light-years across and spinning at close to light speed. Because of its intense gravitation and spinning, it creates a tear in the universe at its center. Ships entering the tear will leave the universe and enter a different one.
See also