Milky Way (Earth-5875)

"This galaxy is vast; its wonders and beauty are almost unfathomable. But the galaxy also hides dark secrets, some of which have lain dormant since the beginning of time itself. There is a danger in secrets, both in seeking and in knowing. Some things are meant to be hidden from view. Some mysteries defy understanding, and sometimes even the things we think we know are untrue. Some secrets should remain untouched. Secrets can tempt your primal instinct, and lead you again into war. The Halos were one such secret."

- Cortana

The Milky Way is a barred spiral galaxy with approximately 100,000 light years in diameter and containing approximately 100—400 billion stars and is orbited by two satellite galaxies, the Magellanic Clouds. It is home to dozens of sapient species, which saw major conflicts through history that shaped the galaxy itself.

Prehistory
100,000 years in the past, the galaxy was led by the forerunner - a now extinct species that had an advanced civilization in the Ecumene, an enormous empire that spanned most of the galaxy.

In 97,745 BCE, the alien species known as the xenomorphs appeared in the galaxy, crash-landing on a forerunner colony, and quickly attacked and infected thousands of Ecumene worlds, leading to the Forerunner-Xenomorph War, where the xenomorphs consumed and killed more than half of the galactic population of sentient species, while the forerunners desperately attempted to study the mysterious foe in efforts to stop their onslaught. The majority of the forerunner leadership decided to activate the Halo Array, an ultimate super weapon that would purge the galaxy of life, including the xenomorphs and themselves. During this period, they built numerous safe havens, such as the shield worlds and the Ark, where they stored the remaining sapient species of the galaxy in a plan devised by the Lifeworkers called the Conservation Measure, later on activating the weapons, which killed all of the necromorphs aside from samples they kept stored on each Halo, and then released the races back to their home planets in the Keyships.

Recovery
100,000 years after, the galaxy substantially recovered, which led to the rise of multiple civilizations, such as the UEG, consisting of most o the human population on Earth, the Covenant, a hegemony composed of multiple different sapient species through the galaxy under faith of the Forerunners as gods. Contact between the UEG and the Covenant was hostile, which led to the First Contact War, all of which killed billions of humans and Covenant species alike.

The discovery of one of the Halo installations led to a series of events that destroyed the Covenant, starting the Great Schism, triggered by the replacement of the sangheili with the jiralhanae, which culminated into an alliance between the sangheili, humanity and the tevarin under the banner of the Swords of Sanghelios in order to stop the Covenant, severely weakened by the loss of their biggest population, which eventually lost during the large-scaled Battle of Installation 00.

Post-war
After the end of the First Contact War, humanity and all other species started a long process of rebuilding. Meanwhile, the Office of Naval Intelligence sought to ensure their former enemies - the species of the Covenant - would not rise against them once more, particularly the sangheili which held the most power of all species in the post-war period. Humanity also advanced in technology with the discovery of forerunner relics during the war.

Sangheili society fell in disarray with the fall of the Covenant, and the species started to engage in a massive civil war known as the Blooding Years, while also fighting against the jiralhanae, who also fell in numerous civil wars, in the continuing Great Schism. Both humanity and the sangheili faced dangers from the resurfacing Insurrectionist movements and the splinter factions of the Covenant, leading to a resurface of forerunner involvement with the awakening of the vex and later the Reapers.

From behind the scenes, the Reapers staged the Second Renaissance, a massive artificial intelligence, a massive rebellion against the species of the galaxy, using legions of vex warriors, successfully invading and disabling technologies across hundreds of worlds through the Milky Way, bringing the species unto slavery.

UNSC
Human colonization through the Milky Way galaxy began in the late 21st century with the settling on the planets and natural satellites of the Sol system, including Luna, Mars, Mercury and the Jovian Moons, in explorations and colonization sponsored by many countries and mega-corporations under the administration of the United Nations. During the 2160s and the 2170s, the Sol system became embroiled in conflicts, with the Interplanetary War being the most devastating of them. The Interplanetary War saw the founding of the United Nations Space Command and the beginnings of the Unified Earth Government.

With the discovery of element zero and the creation of the Shaw-Fujikawa Translight Engine in 2191, humans were able to leave their home system and explore other systems, starting a golden age of interstellar expansion known as Domus Diaspora. The first colony ships were created in 2212, and for the next three hundred years, the UNSC started to colonize and terraform 210 planets, ranging from planets, asteroids, moons, and space stations through the Orion Arm: these colonies became the Inner and Outer Colonies. The Inner Colonies made up the primary military and economic bastion of humanity, while the Outer Colonies were responsible for providing most of the resources of the UEG and the UNSC, despite their lower economic status. The tensions that resulted from the colonization led to a massive civil war collective known as the Secession, which ranged from the ending of the 25th century to the beginning of the 26th century.

After humanity made first contact with the alien alliance known as the Covenant in 2525, what resulted was the catastrophic First Contact War. The Covenant, bent on humanity's destruction in the name of their religion, invaded and destroyed human worlds they could find, killing billions of civilians and military alike in 27 years of war.

Covenant
As the Covenant had been exploring for three millennia, they developed much advanced technology when compared to humanity in the terms of slipspace. As such, they have expanded and explored much larger portions of the Orion Arm and even beyond, in the Milky Way. During its existence, the Covenant encompassed thousands of star systems, and was so massive and complex in organization that entire planets could disappear from its records for decades and even centuries before contact reemerged. Because of limited resources and the vastness of the Milky Way galaxy, the Covenant mostly remained confined to the Orion Arm, much like humanity, with the exception of some expeditions and distant outposts. During their expansion, the Covenant discovered multiple forerunner shield worlds and installations, where they were able to reverse-engineer them in order to create their technology.

While the First Contact War saw the devastation of various human colonies, two Covenant worlds, Saepon'kal and Glyke, were destroyed by the UNSC's NOVA bombs. Balaho, Te, and Palamok, three planets that constitute the Covenant, were both nearly glassed or glassed by the hegemony.

Forerunners
The forerunners explored and colonized the galaxy for millions of years, which granted them to power to easily cross galactic distances and explore it in its entirety. Their empire, the Ecumene, encompassed three millions worlds and colossal installations they built. They were also able to travel outside the galaxy, being able to reach the Magellanic Clouds, the Andromeda galaxy, and built the Ark, outside the galaxy.

Orion Arm
The Orion Arm is a minor spiral arm of the Milky Way, where both the human and former Covenant spheres of influence lie. The Orion Arm was the stage for the First Contact War in the 26th century and the Forerunner-Xenomorph War 100,000 years in the past. It is also the home to Installation 01, Installation 04, and the Excession to the Ark. It is also the home to all known species in the galaxy and their respective homeworlds.
 * Perseus Arm
 * Norma Arm
 * Scutum-Centaurus Arm
 * Carina-Sagittarius Arm

Trivia

 * Art by The-Chronothaur.