Here is the thing..

I am 100% convinced that we are NOT the only life-form out there. If you think of how planets develop and how many other solar systems are out there - its ABSOLUTELY possible for there to be other life forms.

Secondly - who says carbon based life forms have to be the only form of life? What about gas based life forms, light based life forms ect. Just because we have not scientifically explained it, doesnt mean it doesnt exist, just means we havent proven it yet or found a way to explain it.

Have they been here - I have no way of knowing. I lean towards yes, but I also think if they are smart enough to get here they are smart enough to be disguised or blend in, or make people forget. If they are compassionate beings, as an evolved being might be, they would realize that we as a society are not ready to beleive fully in aliens and to do so would lead to mass confusion and panic. I think that when we are ready, they will come.