So this theory is totally out there, but it just came to me while playing ME3. What if there were other intelligent, peaceful species out there that have already formed alliances/pacts and live together in peace? And what if the different races kept themselves hidden until they allowed themselves to be discovered, by waiting until humanity had reached a maturity and was able to finally work as one? And once we found them, they'd share their technology, planets, etc?
Crazy...I know. Just a thought that I thought would be fun to share.*