the government covers up lots of things. Also, one could argue that they let you know what they want you to know. There was a lot of details recovered from the infamous Roswell crash site, and most of the evidence points to people not wanting other people to know about things. It is completely possible that another planet could have began their evolution and be advanced enough to travel through space, and then, visit other planets.
A long time ago they probably argued that the world was flat with "statistical probability".