Anyway, I used to have a plastic boomerang. If thrown correctly, it would return - basically. Out of all the times I threw it, it only returned to where I could actually catch it maybe twice. Then. It got stuck in a palm tree and I had to find something else to do.
The best thing you could read to catch up is apparently ROM the Spaceknight, since that basically covers the whole plot. No story with any of the main characters of the Annihilators is a factor.
All you need to know about the previous arcs mentioned is that Starlord created the Guardians of the Galaxy, but what the universe needed was the Annihilators. It's all explained in Thanos Imperative: Devastation one shot, so you could pick up that if need be.
@rapest_of_super_heros: You are correct that it isn't an entity in the 616 universe. Whatever the sentience of the universe was, it bonded to Galan to become Galactus of the 616 universe. Together, they saved themselves from the previous universes destruction to become a new being.