They are here now.
One could be in the room with right this minute and you would have no way of knowing it.
They just don’t play by human rules.
Yeah? And what are they doing? Why are they here? And they're invisible?! Wow, I didn't know there were invisible aliens (or maybe you're going with the Carl Sagan 3-D creature visiting a 2-D world bit?).
But ok, let's assume there are invisible (or extra-dimensional) aliens here. Clearly, they are so advanced that there's nothing I (or anyone else) could do about them. So until they decide they want to show themselves, ask some questions, or round us all up as guests (or food!) at their next intergalactic barbecue, there isn't much point in my worrying or losing sleep over them, is there?