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That's, uhh...quite the revelation there. In short:
Humanity was very technologically advanced, but of course they had to go bomb the Earth and make it inhabitable.
After that, they colonize other planets and decide to get rid of excess humans by sending them on colonizing missions.
Anne, the first of the Star Queens, ends up on a planet similar to Earth (including dinosaurs!) and makes a deal with humanity's HQ that they'll play nice with the environment or get eliminated.
Of course, humans can't stay civil for long, and the neighboring nation of Brigion is threatening to conquer Graal and tear down the wall that the spaceship's supercomputer had put up. Since Anne pleads with HQ to not uproot the people who've gotten attached to this planet, it makes sense why this knowledge would be sealed away.
Which seems like a silly scenario, because you could have 9 billion people on the planet, in dozens of states, and things will be fine as long as Graal doesn't get attacked? What about Brigion who'd been supposedly expanding their territory through war?