Is it just me or is the AI not that aggressive? Granted I am only on my second game but so far it seems that even when the AI army power is considered the same as my own it just doesn't send fleets into my empire. Have you seen the AI launch major attacks against you?
EDIT: I should also add that 2 large AI empires are at war and have been so for some time yet I don't think I have seen any planets change hands.