Closest thing to an "official" storyline is just the stuff that's in the various racial descriptions, and most of that is purely background prehistory (the Drushocka/Eee rivalry is the only counterexample that comes to mind).
It's probably for the best, because unless you're an ancient race how do you explain having a long history of interaction with some other species that you haven't actually met in your current game yet? Not to mention the way your technology and population keep mysteriously being reset to your starting homeworld...
I think each game is a story unto itself, without much in the way of outside referents.