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 orokin colony, and quickly attacked and infected thousands of Ecumene worlds, leading to the Forerunner-Xenomorph War, where the flood consumed and killed more than half of the galactic population of sentient species, while the orokin desperately attempted to study the mysterious foe in efforts to stop their onslaught. The majority of the orokin leadership decided to activate the Halo Array, a ultimate super weapon that would purge the galaxy of life, including the flood and themselves. During this period, they built numerous safe havens, such as the shield worlds and the Ark, where they stored the remaining sentient 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 races through the galaxy under faith of the orokin 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 Human-Covenant War, humanity and all other species started a long process of rebuilding.