The gi'ratIMAK came in the midst of the 22nd century to do us a favor. They wanted us to live in harmony with the universe, which in their conception of things meant that 5/6ths of the human race must die, and the rest would serve the gi'ratIMAK as personal servitors because using machines was evil.
However, using mental powers was a-ok to the gi'ratIMAK. But it did not provide enough, so they found they had to find slave races to take over to work for them. Otherwise an Enlightened Being might have to sully his mind with actual thinking about work.
Humanity took the new ideas about psionic power to heart, and their own tech, and fought back. The slave race, the Stol'cazm, are ten feet tall, naturally armored, move as fast as a mongoose, and bloodthirsty. The Blood Brigades took three years to reduce Earth to a charred cinder. Thus the environment was saved.
Earth had fought as hard as it could behind fixed defenses in the Outer System, and then the Earth and the Moon fought. Meanwhile, a thousand, thousand ships set out. They were powered by designs given to us by the Rip'ti'tor who pretend very hard to be loyal to their masters while subverting them every chance they get. These antimatter powered hyperdrive ships faced one problem.
No one had licked FTL communications. And without communications, the Grand Diaspora was licked before it began. No one ship had enough knowledge on its own, enough ideas on its own. Plus, no one ship could guess when suddenly a psi-powered Starcracker battleship might appear out of hyperspace and smack them into dust.
But if the Ships could talk to each other, then they could compare ideas. And stealthy scout ships could keep an eye on the enemy battleships, and send orders for those who needed to hide or flee to do so.
All that was required were thirty thousand astral travellers. Telepathy did not work in this world, and for Humans, telekinesis was unreliable. But with the proper trance state, and drugs, Humans could astral travel at FTL, and deliver and receive messages in a great Net between the stars.
And so the guerilla war between the Ships and the Enlightened Ones continues....who knows what will happen. Maybe a human will find an ancient alien race's tech, or the Enlightened Ones will develop a means to detect stealthed scout ships? Maybe the Ri'pi'tor will be exposed, and come out to fight? Will the Stol'cazm ever put aside their unquestioning loyalty? And what about the dozen or so other races that slave for the masters?