Wesly, Unknown is NOT a player, it is what happens when someone goes idle for more than a week, the civilization becomes an 'unknonw' much like a spliter colony in moo, except you still have to conquer it.
I just had my first loss, I control all inhabited planets in my system (either they are mine or I have them under seige) but a goof named pyrox has put a fleet of his over one of my free planets. No no... it's MY system. So sent a fleet of 15 destroyers to check it out... they are no more, but I took out 40% of his fleet. Anyhow, he has 68 destroyers coming down his throat now.