This subject has been a very interesting and mysterious topic, and over the course of history there has been a lot of clues. Such as multiple UFO sitings, paintings, and objects.
The Egyptians have left several "clues", like the Pyramids. The Egyptians were primitive people that lived 5000 years ago, making it near impossible for them to accomplish such a feat, seeing as how each block weighed about 2500 kg.
This painting shows two figures, they have elongated skulls which look very familiar to those aliens you see in movies, they also have large abdomens and thighs.
This is a picture of Nafertiti the Goddess who lived in Egypt, she looks very similar to the figures in the picture before. I recently saw the new Indiana Jones movie, which had a Crystal Skull with a elongated Cranium like these one. The skull in this next picture could have been a human skull, the people could have possibly saw a alien(s) with a head shape similar to the ones in the picture's above, so they decided to purposely shape there skulls to represent the alien's skulls to honor or be like them, because possibly the aliens built the pyramids for them with advanced technology.
Also the USA government possibly could be hiding the existence of aliens.
Here's a video for possible proof:
Let me know what your guys' opinions on this topic is, or what your government thinks.