The Imperium has has come to Humanites diminsion to harvest it for what they call Dark Matter. They have sent a simple message with not their demands but a proclamation:

To the Meat sacks of Earth in Dimension 4421425. Your dimension has been discovered to contain large amounts of Dark matter and the Imperium has laid claim to your dimension and the resources therein. In the great benevolence of the mighty Imperium and not at all due to our scientists discovering your species has great potential for evolution we have dispatched several Arks or great space vessels that will carry humanity to its new home. Fear not, although your dimension will be sucked dry most of you will live. Nanite permeation of your species shows a survival rate of over seventy percent. Once on the Arks if you do not wish to relocate, we will recycle your biomass to serve the Imperium and bring it to greater glory. Long live Emperor Dius XII, may you eternally revel in his glory.

Those who survive nanite processing must discover how to live in this new world provided to them. Form new alliances and discover everything the Imperium has to offer. Discovery and exploration is always rewarded by the System that governs the Imperium.