Milky Way

The Milky Way, also known as the Milky Way Galaxy or simply the Galaxy and known to the Covenant as the Great Spiral among several other names, is a barred spiral galaxy. It is estimated to be around 160,000 light-years in diameter and contains 100—400 billion stars, and at least twice as many planets. The Milky Way has several satellite galaxies and is part of the Local Group consisting of several galaxies, which form part of the Virgo Supercluster, itself a component of the Laniakea Supercluster. At the radius of Sol, the galaxy orbits around itself in roughly 240 million years in what is known as a "grand cycle" or galactic cycle.

Humanity, the Covenant and its peripheral civilizations, along with various other cultures, known as the Ulterior, inhabit the Milky Way today, whereas the Forerunners were its most notable historical occupants before their disappearance around 100,000 years before the present.

History
The Milky Way has a long history of habitation going back hundreds of millions of years, as evidenced by various oddities such as the existence of various ecological anomalies with lifeforms discovered light-years away from their original homeworlds. The abundance of life in the galaxy, both now and in the past, has also given rise to theories about teleological panspermia carried out by actors unknown having given rise to galactic life. It has also been hypothesized that "islands" of phosphorus may have been artificially engineered across the galaxy in primordial times to facilitate the development of life, along with various cases of possible "sculpting" of planetary systems to be more hospitable for life. Such theories remain in dispute, even as the existence of ancient alien civilizations in and of itself is an incontrovertible fact.

The Forerunner were the last major galactic civilization to rise to galactic predominance between 100,000 and 200,000 BCE. Their civilization would ultimately fight a losing war against the Flood parasite, culminating in the activation of the Halo Array, a network of superweapons that sterilized the galaxy of all sentient life to rid the Flood of potential hosts. While this ended the Flood threat, it also snuffed out a large number of developing species, and despite the Forerunners' efforts to preserve galactic biodiversity, it is estimated that only a fraction of biological species survived the indexing, reseeding and subsequent reacclimation.

Galactic Center
The center of the galaxy is the supermassive black hole and radio source known as Sagittarius A*. The most conspicuous feature of the core region is a bar-shaped structure of gas and intense star-formation activity (known to the Covenant as the Fount). The bar is surrounded by the Five-kiloparsec ring, a dense cloud of molecular hydrogen which is likewise home to much of the Milky Way's ongoing star formation. Due to abundant radiation present across the core, the entire region is inimical for biological life, while the density of dust and young stars makes it virtually impossible to navigate via slipspace.

Spiral arms
The galactic spiral arms are elongated strings of stars, gas and dust that slowly orbit the galactic center. Galactic civilizations typically develop along the length of the spiral arms as the greater density of matter therein makes slipspace travel and navigation easier.

Near 3kpc Arm
The Near 3-kiloparsec Arm is one of the two minor arms orbiting around the galactic core. Rather than separate spiral arms, the Covenant identify the 3kpc arms as a single elliptical structure known as the Inner Girdle.

Far 3kpc Arm
The Far 3-kiloparsec Arm is one of the two minor arms orbiting around the galactic core. The Far 3kpc Arm is identified by the Covenant as part of the elliptical structure known as the Inner Girdle along with the Near 3kpc Arm.

Sagittarius Arm
The Sagittarius Arm, also known as the Carina-Sagittarius Arm and the Raan Spine to the Covenant, is a lesser spiral arm located coreward from Sol.

Orion Arm
The Orion Arm, also known as the Orion-Cygnus Arm, Orion Spur or the Local Arm and known to the Covenant as the Raan-Uchaal Bridge, is a minor arm located between the Sagittarius and Perseus arms. Both the Human Sphere and Holy Ecumene reside here.

Perseus Arm
The Perseus Arm, known to the Covenant as the Uchaal Wing, is one of the galaxy's two major arms, beginning from the distal end of the Galactic Bar.

Scutum-Centaurus Arm
The Scutum-Centaurus Arm, also known as the Scutum-Crux Arm and known to the Covenant as the Iarben Wing, is one of the galaxy's two major arms, beginning from the proximal end of the Galactic Bar.

Norma Arm
The Norma Arm, known to the Covenant as the Niem-Shabr Spine is a lesser spiral arm that begins from the near end of the Galactic Bar and loops around the Perseus Arm as the Outer Arm.

Galactic halo
The galactic halo is a diffuse spheroidal mass of old stars, gas, dust and dark matter that surrounds the galaxy.

Humanity
Up until the mid-26th century, humanity's sphere of influence mainly encompassed the Local Bubble, a vacuity in the interstellar medium surrounding Sol. Since the end of the Human-Covenant War and the development of more efficient slipspace drives, human civilization began to gradually spread further out, with most expansion efforts by the UNSC being headed spinward to the Cygnus Verge and the coreward Aquila-Sagittarius Region, where sanctuary-colonies were also established by unaffiliated actors during the war. Some human groups settled along former Covenant ingress routes at the Pleiades Corridor and Tempest Crossing, particularly after the onset of the New Diaspora; these colonies would be shaped by the proximity of the Covenant, and form the first "melting pots" between humanity and the wider civilization of the Orion Arm.

The Covenant
The Covenant meta-civilization occupies large swathes of the Orion Arm of the galaxy, stretching over 7,000 light-years from the Local Region toward the trailing side of the spiral arm and along its entire width in many areas. The Covenant also had remote outposts in the Sagittarius and Perseus Arms, and with their advanced slipspace drives, expeditions even further away were possible. High Charity or any large and well-equipped Covenant ship could, with some preparation, circumnavigate the entire galaxy in a few months. Indeed, High Charity occasionally did, as did some armadas sent out periodically to seek for the "Sacred Rings". However, the challenge was not moving about the galaxy itself, but actually finding anything of value; without proper star charts or even clues, all the Covenant would usually find were empty systems. As such, most search efforts were focused on the Orion Arm, where expeditions could be supplied on a regular basis by permanent colonies and the grand supply routes of the Arterial Network. When galactic circumnavigation expeditions were arranged, entire armadas would typically set off for decades if not centuries, so that they may carry out a thorough search; such expeditionary armadas would not usually return to the Holy Ecumene before they had found something worth bringing back.

Ulterior civilizations
Not many external civilizations were known to the Covenant, though this is hardly surprising; even with their resources as a civilization devoted to finding the Halos and other Forerunner relics for 3,400 years, the Covenant are estimated to have surveyed less than 0.001% of the totality of the galaxy's star systems. It is possible that veritable interstellar empires exist outside the Covenant's knowledge in less explored areas of the galaxy, with smaller cultures such as the pre-war humanity inhabiting small pockets of space that may still go undiscovered for thousands of years. Still, a few Ulterior civilizations were known, with the Others likely being the largest one, whereas little is known of the current status or extent of the Gryunjalla, Optem or Jehioi. Several others were hypothesized based on various kinds of second- or third-hand evidence, though little was known about these.

Extragalactic space
Beyond the galactic halo, extragalactic space is effectively impossible to navigate or traverse via slipspace due to both the absence of mass shadows to "fix" onto or for drives to gain "traction" on. The only known extragalactic location to be visited by the modern civilizations is the Forerunners' furthest outpost, Installation 00 or the Ark, located around 260,000 light-years from the galactic center at a slight angle to the galactic plane. The Ark can only be accessed by an incredibly advanced slipspace portal network of Forerunner origin, the only known remaining portal of which is located on Earth, buried there in the final days of the Diluvial War by the Librarian.