There are beings which exist outside the human eyes' visible light spectrum. They are only a fraction of the invisible hard evidence of the aliens existence.

Let us think for a moment about alien, regarding possibility of the existence of other civilization beyond our dear planet earth. We probably conclude that it's all just hoax, or some rumors issued by ruling government for some purpose. Now, we can contemplate a bit about it by asking this simple rhetorical question, 'How could possibly people be able to think about something unknown, which have never been popped out in human minds at all?'

You, me and everyone else might be able to think and say one simple word, 'alien'. Just curious, when that word crossed in our minds and said, we are all refer to the same object.

There are many things which can not be expressed by words.
There are many words which can not be spelled by human tongue.
There are many tongues which utter one single truth.

The 'element of surprise' of the alien invasion will cause a certain event in human side which is called the 'moment of shocking'. This will be an advantage point to the invaders, a weakness point in human side. This kind of situation, can only be overcome if and only if, some protocols of counter-invasion have been made before.

For the sake of popularity, vote, and reelection, most governments remain secretive to the public about the truth of alien contact with mankind. For them, it is better like that than being thrown from the throne.