What makes you think they have only been to the moon! Black Box projects have been going on since the cold war and it's no secret they understand more about gravity than they care to admit. Last I read there was over 4000 classified patents and since 911 they have tightened control even more. I remember seeing a video on you tube with an interview with a senior scientist from Lockheed Martin where he showed some experiments that twisted the laws of physics.
I personally don't think the earth is the only life bearing planet out there, not when our galaxy alone contains billions of stars - let alone the billions of galaxies out there.
If roswell was true, and there is significant evidence of a cover up and also ongoing visual encounters from very reputable people, some of which where in significant positions of responsibility, then they have had over 50 years to back engineer their craft.
So back to my first sentence, forget the moon, where have they been that we don't know about?