Milky Way Galaxy

Total star systems
 Size
 Galactic Arms
* Perseus Arm
* 3-kpc Arm
* Norma Arm
* Scutum–Centaurus Arm
* Carina–Sagittarius Arm

Minor Arms:
 Globular Clusters
 Quadrants
* Beta quadrant
* Gamma quadrant
 Dwarf satellite galaxies
* Large Magellanic Cloud
* Small Magellanic Cloud
* Canis Major Dwarf
* Sagittarius Dwarf Elliptical Galaxy
* Ursa Minor Dwarf
* Sculptor Dwarf
* Sextans Dwarf
* Fornax Dwarf
* Galactic core
* Inner circle
* Core region
* Attican traverse
* Frontier
* Outer colonies
* Inner colonies
* Orion complex
* Outer council region
* Inner council region
* Outer veil
* Terminus region
 Major trade routes

The **Milky Way galaxy** was one of the billions of galaxies that existed in the universe. This galaxy was the setting of the histories of the Citadel, the Ecumene, the Precursors, the Reapers, and countless other species and civilizations.

The Milky Way was 100,000 light years across, or 30,000 parsecs (a parsec is 3.258 light years), 15,000 light years deep at the galactic core and approximately 13.2 billion years old. It was a barred spiral galaxy that laid within the local group of galaxies neighborhood, which is just a very small portion of the universe. A supermassive black hole existed at the center of the galaxy. The galaxy was orbited by nine satellite galaxies, most notably the smaller and larger Magellanic Clouds. However, most of the satellite galaxies were described as having ancient, metal-rich remnants of stars with little life. During the Citadel-Era, the galaxy was in the process of absorbing a dwarf galaxy SagDEG. The Milky Way's nearest large neighbor was the Andromeda Galaxy, some 2.5 million light-years away. There was also a subspace barrier beyond the edge of the galaxy that made slipspace, warp and mass effect routes outside the disk almost impossible. The galaxy had nearly two hundred or more globular clusters.

While there was a subspace disturbance outside of the galaxy, faster-than-light modes of transit such as the mass relay network and the much slower slipspace and warp travel made it possible *within* the galaxy to have an enormous, and diverse, civilization. There were approximately 400 billion stars, and around 150 billion of these had planets that could support life. Ten percent of those planets developed life, while sentient life developed in 1/1,000 of those (about 20 million). Factoring in the output of heat and light needed for an advanced civilization to form, there were 7.1 billion truly habitable stars within the Milky Way, and about 3.2 billion habitable star systems. However, it was estimated that about one billion of those systems were actually populated. During the days of the Federation more than 7 million systems met the requirements for Citadel representation, and 1.75 million planets were considered full member worlds. The population, of the nearly 40 million systems that the Citadel was aware of, amounted to more than 60 quadrillion beings.

The Milky Way was naturally subdivided into a number of "arms", with the Federation, the Covenant Empire, and later the Covenant Remnant along with Yautja worlds mostly occupying the Orion Arm. Other arms are occupied by other stellar empires, such as the Klingon Empire, the Romulan Star Empire, the Borg Collective and the Geth. Almost all other regions are under the jurisdiction of the galactic entity known as the Citadel, which is located near the galactic core. For the sake of standardized navigation the Citadel and most other galactic powers divide the Galaxy into four equal segments known as quadrants.

Given the huge size of the galaxy and the limited velocities available at warp speed a number of races use networks of mass relay pathways to navigate the galaxy. These mysterious mega structures where supposedly built by the mysterious Prothean race, and allow almost instantaneous faster-than-light travel along a set path. While most mass relays are used in the governing of Citadel space, some are formally controlled by a variety of other empires or races. These include the Vaadwaur's routes, and the Borg Transwarp, Iconian Gateway and the Caeliar warp space tunnel networks or the Bajoran wormhole.

## History

*Main article: Timeline of galactic history*
### Creation

The Milky Way galaxy was created in approximately 13,000,000,000 BC from a large expanse of closely-packed stars, which coalesced to form an elliptical galaxy. In time, it's familiar spiral shape arose, just a billion years after it's formation. The C'tan, or Star Gods, soon came to inhabit it.

### Pre-Precursor civilizations

Before recorded history, the first galaxy-spanning political entity was the aptly named First Empire of the Precursors, which ended a billion years before the so-called Citadel Era. They disappeared under mysterious circumstances, creating the artificial super-intelligence known as the Catalyst to solve the problem of organic and synthetic life. This led to the creation of the Reapers and the Cycle, which would endure indefinitely until the Catalyst could determine a better solution.

### Pre-Prothean Times (1,000,000,000 B.C. to 250,000 B.C.)

The Catalyst is the original creator of the Citadel and the mass relay network. These massive constructs exist so that any intelligent life in the galaxy would eventually discover them and base their technology upon them – all part of a scheme to harvest the galaxy’s sentient life in a repeating cycle of purges. Engineered creatures called keepers were placed on the Citadel to maintain the station during their absence and to open it for them when they decide to invade. Whatever races became space-faring would discover the mass relays, and accelerate a galactic society that would eventually become harvested.

It was also around this time when the Old Ones were created by the C'tan and their galaxy-spanning empire reached its peak circa 60,000,000 BC. The Necrontyr would soon develop in the outer reaches of the galaxy. Upon encountering the Old Ones, the Necrontyr, with there incredibly short lifespans, were furious at the Old Ones' refusal to share the secrets of their immortality. But as time went on, the Necrontyr race grew divided. Various dynasties fought for control in the Wars of Secession. The Triarch, the Necrontyr ruling council, realized that the only hope of uniting their race was with an external threat.

The Necrontyr Silent King would use the Old Ones' refusal to share immortality with the Necrontyr as a pretext for war. This war, the War in Heaven, would be a devastating galactic conflict. The Necrontyr began to lose, badly. They had started a war they could not win. The Nectrontyr race began to fracture again, in a second iteration of the Wars of Secession.

It was during this time that the C'tan, or Star Gods, first blighted the Necrontyr. The C'tan known as the Deceiver was the first to make contact with the Necrontyr. The Deceiver spoke to the Silent King of a war between the C'tan and the Old Ones which the C'tan had lost. The Deceiver proposed that, in return for the Necrontyr aiding the C'tan against the Old Ones, the C'tan would give the Necrontyr immortality.

The Silent King agreed, and it was not long before the Necrontyr were worshipping the C'tan as the Star Gods. Armed with weapons and advanced technology that would allow the Necrontyr to stand a chance against the Old Ones, the Necrontyr prepared to begin their war anew. However, there was one thing the Necrontyr were willing to postpone their war for.

### The Ecumene (249,000 B.C. to 200,000 B.C.)

By 249,000 B.C. the Prothean race formed a modest interstellar empire on the Orion Arm of the galaxy, utilizing slower faster-than-light methods of travel such as warpspace. It is during this time that they discover the mass relay network, and use it to colonize the rest of the galaxy. Within a period of about 3,000 years, they have colonized millions of fertile worlds and discovered the Citadel, effectively becoming the dominant civilization in the galaxy. Their subsequent empire becomes known as the Ecumene.

Early in their development, the Protheans encountered a hostile machine intelligence which threatened to overwhelm them. To defeat the machines, the Protheans decided to unite all of the galaxy's sentient organic life under their empire. The other organic races were free to resist, but those that tried were crushed, and none ever managed to best the Protheans' might. In time, each of the subjugated races assimilated into Prothean culture and came to think of themselves as Prothean. United under a single cause, the Prothean Ecumene successfully held off the enemy machines in a conflict known as the Metacon War.

In about 200,000 B.C. the Reapers returned and initiated a purge against the Ecumene. The Protheans tried to resist like so many others before them, only to fail due to the Reapers' insidious might. Eventually, the Protheans once again called upon the aid of all the other races in the galaxy. The Reapers, never having faced a civilization that united the might of all of the organic races in the galaxy, were resisted, in the First Reaper War.

The Catalyst, concluding that the frequent disharmony and warfare of organic life was the source of the problem, created the Flood as a biological weapon that would end all biodiversity, and thus "pacify" the races of the galaxy in response to their unification in the First Reaper War. The Reapers were no longer as effective as they had always been, and the Flood's virulent nature would be especially deadly against organic life and their simple, protein-based bodies.

### Decline of the Ecumene and the rise of the First Men (199,999 B.C. to )

By 150,000 B.C., the race known as the First Men had achieved a considerable level of technological sophistication, achieving interstellar travel and colonizing planets along the Orion Arm of the Milky Way galaxy. They became a major political and military power rivaling the Protheans; indeed, the Didact considered them the second greatest military power in existence and the latest challenge to Prothean influence. The First Men would eventually join the Citadel and become a Council Species.

The Ecumene has declined significantly. Various species, seeing the threat of the Reapers as a thing of the past, have begun to retreat or resign from the Council and Citadel, forming their own governments and empires in different sectors of space around the galaxy: the entire galaxy is no longer united under one banner. During this time, the Protheans themselves begin to technologically stagnate, while the First Men, a comparatively young race to join the Council, are advancing quickly.

Dwindled to only include a handful of races, with many other galactic governments existing in varying degrees around the galaxy, the Ecumene is all but gone. The Protheans are still the pinnacle race, but many have questioned whether they are still fit to bear the *Mantle of Responsibility*, their founding beliefs.  In 110,938 B.C., the Catalyst creates and deploys the Flood, preparing for the next iteration of the Cycle.