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Nightfighters as depicted on the cover of a Xeelee Sequence novel

The Xeelee are a hyper-advanced species from Stephen Baxter's Xeelee Sequence. They were first remotely mentioned in the 1986 short story "The Xeelee Flower", 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 other space-faring civilizations. They have demonstrated the ability to routinely construct Closed Timelike Curves and have explored time travel to engineering their 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 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 autocatalytic 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. Within a Planck epoch (10⁻⁴³ seconds) after the Big Bang 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, the universe took on a fundamentally different character and there was a corresponding mass extinction, but small enclaves of intelligence 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 autocatalytic reactions in the new baryonic matter, thus emerging 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 reaches back to within minutes of the big bang.

The Xeelee use primordial black holes as habitats, construction tools, and computing devices because, at the time they emerged, the event horizon was one of the few things 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 super-massive 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 purposes. Over the billions of years since their migration, the Xeelee have become a ubiquitous part of the complex ecology of the supermassive 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 hinted that 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 entities that prefer habitats in the gravity well of stars. In general, the photino birds are probably unaware of almost all other forms of life since 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 stopping nuclear fusion in the cores of stars, thereby prematurely aging them into white dwarves. The resulting dwarves will have the same gravity well but are stable for billions of times longer than their progenitor. Because white dwarves 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 the 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 were ultimately unsuccessful, as the Photino Birds could operate outside the universal constraints imposed even upon the Xeelee. Most critically, the Photino Birds were not bound by time so they could instantly work around any Xeelee strategy and replenish any losses. When the Xeelee traveled back in time to shortly after the Big Bang, the Photino Birds were already there, having traveled back to the beginning from the end of the universe. This meant the Photino Birds could outmaneuver the Xeelee through time, making them impossible to displace. This revelation ultimately led to the Xeelee constructing the Ring to flee from a universe they could not save.

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 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 did not know about the Photino-Xeelee war or even what was at stake, making the Xeelee seem inherently 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 there, lacked an obvious explanation. Though there were several orders of magnitude fewer Xeelee than humans in the universe as far as is known, their high degree of technological 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 for 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. Furthermore, very little effort was directed toward projects that did not have an immediate and conspicuous benefit to maintaining the massive logistics 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 out-think 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. The Xeelee, however, demonstrated several instances of compassion and even charity toward other species, particularly when it involved escaping the Photino Birds' destruction of the 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 humans, the Xeelee constructed a universe for the Silver Ghosts that was appealing to their mathematical nature. Before their retreat from the universe, the Xeelee left ships as 'lifeboats' 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 after having 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 opposition to the Xeelee. It further implied that the Anti-Xeelee, in at least some regards, is traveling backward in time like some anti-particles are thought to do.


The Xeelee were masters of time and space. They crafted their starships, known as Nightfighters, out of a strange form of matter. The material is incredibly sharp, hard, and transparent; due to its unique space-time construction, it has strange gravitational effects.

Construction Material[]


Starbreakers are beams of coherent gravitational waves that the Xeelee use as both weapons and tools. They are consistently described as thin beams of cherry red light that emit from Nightfighters and other Xeelee vessels encountered around Bolder's Ring. In the short story, Blue Shift the source of the Starbreaker is described as a pistol-like device designed for a hand the size of a human child. There is intricate wiring around the handle and an adjustment knob on the back. It will emit synchrotron radiation on the lowest setting, suggesting that it is powered by some form of particle acceleration.

Sugar Lumps[]

Nearing the end of their work on the Ring, roughly 4 million years from the present, the Xeelee constructed several hundred enclosures to return to the beginning of their history 13.5 billion years ago. These enclosures, called Sugar Lumps, appeared as enormous glowing cubes varying in size from a few thousand to tens of thousands of kilometers on a side (capable of enclosing moons or planets). More than a simple Closed Timelike Curve, the Sugar Lumps are described as having a negative vector in time (i.e. traveling backward in time via a time symmetry) and function as inverse time capsules. When the Sugar Lumps arrive at their destination in the distant past, the Xeelee would emerge as a fully developed race with their knowledge of future events and be able to immediately initiate their grandest projects against the Photino Birds.

The Sugar Lumps are guided both in management of their travel and coordination of their effort, by the Anti-Xeelee. Use of the Sugar Lumps enabled the Xeelee to consciously manage their evolution as well as make any necessary edits to the history of the universe so that their projects are unopposed to the maximum extent possible. It is therefore likely that any species which made a substantial rivalry with the Xeelee were prevented from evolving in the first place, and further that the Xeelee were unable to prevent the evolution of the Photino Birds: likely because their nature of being unbound by time meant they were already firmly established even then.


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.

The ultimate goal of the Ring is to allow baryonic life to escape the universe, as the Xeelee ultimately determined that they could not stop the inevitable victory of the Photino Birds. As masterful as they were with their manipulation of time and space, the Photino Birds were completely unbound as the Xeelee in regards to time so could outmaneuver them.