From the sight posted by Azadre... Thus, they are physically invisible from man as their description suggests. This invisibility is one of the reasons why some people have denied their existence. Yes, invisibility usually makes people believe something is not there.
Azadre -- i'm really curious why the Koran always says "We" instead of "I"....like "when we created the world..." who, according to the Koran, was a creator?? i may be grossly misinterpreting this, so please forgive me if i am! I just know that Islam preaches the radical unity of God....and We doesn't seem to fit with that.
source Also, the Koran is intresting in that its all dictated by god to Mohammed from, I believe, a burning bush or something? An intresting contrast is the bible, which is more an amalgamation of many disparate documents. What this implies of says, I have no idea, but it's intresting nonetheless.