[quote=Sep]
How do you know they aren't some new group who appeared in your galaxy and epickilled your Emperor?
[/quote]

Because if they did then the Chaos gods would now be free from The Warp and would set about unmaking existance. So, the fact that they are all still alive proves that the Emperor isn't dead, plus... Seriously? Terra's defenses are absolute, blood hasn't been shed on the planet since the Horus Heresy, no fleet has got anywhere near the Holy Mother since that time, I don't think a fleet even can, due to the Emperors powers pushing any attempted raiding fleet off course.