From what I understand- scientifically it is possible and even likely that other life could exist in the universe. However, the chances of them evolving to a point where we could communicate at the same time in the history of the universe and in close enough proximity to transfer information makes it nearly impossible that we will ever encounter them.
I wouldn't say that the testimony of guys who have been cooped up in a tin can, deprived of oxygen, sleep, and subjected to severly high g-forces is entirely acceptable as proof of extraterrestrial life or government coverup.
The government has never really been able to successfully cover anything up.