Exodus

The Exodus (sometimes referred to as the First Wave of Colonisation) was a historical event leading to the abandonment of Old Earth and the eventual human colonisation of the Achelois Galaxy.

Lead-up
In 2068 Common Era, the Långsyn Space Telescope detected abnormal energy fluctuations originating from the binary star system IK Pegasi. Further investigation by the Space Affairs Council's extrasolar probe network led to the discovery that IK Pegasi was in the throes of supernova, and would reach Old Earth in approximately fifty years. Simulation and modelling predicted that humanity would survive the initial wave, but the resulting destruction of the ozone layer would render the planet vulnerable to intense solar radiation, leaving little-to-no life remaining within a century.

Preparation
Amidst rampant climate change, rapidly escalating natural disasters, and a pandemic caused by a rogue biological weapon, this announcement barely made a dent in the public consciousness. Among governments and corporations, however, there was great concern. The 2071 Exodus Treaty (for which this period of time is now known) resulted in the creation of the Commission for Human Preservation, a multinational organisation. The C.H.P. pooled funding from various sources, including harsh taxes and gutting social programs, to design a series of ark vessels called Saviour Ships.

Various nation-states and corporations used the open-source designs to construct their own Saviour Ships, often tweaking or modifying the designs to fit their specific purpose. Unfortunately, riots and civil unrest caused by the great expense required to build these vessels forced several smaller nations and corporations to abandon their projects. These half-finished designs were often amalgamated by larger, wealthier powers. Roughly six thousand Saviour Ships were constructed over a period of forty years, each with a capacity between 500,000-750,000 persons.

Departure
On March 18, 2101 Common Era, the first group of Saviour Ships, administered by the Scandinavian Treaties Organisation, departed the solar system. Over the next four months, all of Old Earth's Saviour Ships left, with the last vessels of the West Saharan Alliance departing on August 3, 2101 Common Era.

The Saviour Ships eventually (estimations for time between departure and arrival range from a few thousand years to several million) arrived in a region of the Achelois Galaxy now called the Known Human Territories, an area which the Artificial Intelligences controlling the vessels predicted to contain an abnormally high amount of suitably habitable planets. Following nearly a century of isolationism, wherein the passengers of the Saviour Ships rebuilt themselves, interstellar communications were eventually established, leading to a period of exploration and trade known as the Second Wave of Colonisation.

Summary
Historians estimate that only about a quarter of Old Earth's twelve billion population was able to secure a position aboard the Saviour Ships, leaving nine billion individuals behind on Old Earth, with mere years before supernova impact and no real governments or power structures in place. With these circumstances, and the estimated timescales for departure vs. arrival in the Achelois Galaxy, experts confidently conclude that humanity in the Milky Way Galaxy is long-extinct.