Alien, are not very well known, is true, and we must think that by math chances, there must be much more then 1 alien species.
-Some of them can be bad, others good. - They already studied our DNA and much more, that they know we are afraid of them, and they simply dont want to show up yet, they Hide, and they also, probably are invisible when they need. So we dont see it. Also our bacterial world is different from them, so they cant contact us , wihout infection and possible have no immune system for our world.
