I saw an interview with a famous scientist once, I think it was Michio Kaku. He said the reason that other life forms that are more advanced than humans have not revealed themselves to us is because it is not worth it to them. He said it is basically like when we walk by an ant hill, do you get on the ground and start talking to and communicating with the ants in the ant hill? The answer is obviously no, he said this is probably similar to how much more advanced a living species of aliens, who is capable of intergalactic travel, is to humans. So basically it is possible they know we are here, but they just don't care, we are just an ant hill to them...