Navy: UFO files could cause “grave damage” to national security if released
msn.com
msn.com
The second least-cost conspiracy is that like 'Corona' with the NSA and donation to NASA, revealing original quality material informs hostile entities of the resolution of current detection and recording devices in ways the Navy do not wish to reveal, since it will alter the advantage in imaging and detection. This is almost the inverse of above: they did detect it, thats revealing in ways they don't want people to know.
The most expensive-cost conspiracy (i.e. what i feel is the least likely to survive occam's razor) is "Aliens"
Now consider its 43 years later with computing technology power that was beyond the dreams of the engineers working on stealth in 1975-1977 and with US military budgets that have been at times larger than the rest of the world's spending put together.
I'd expect that the current state of the art experimental US military tech would be considered by many to be "out of this world".
Not to mention if these guys are bad, it's for our collective advantage to keep the knowledge restricted to the absolutelly minimal necessary people to manage and prepare the rest for the really bad news.
We're living in a world awash in decades' worth of science fiction media and pop cultural references to aliens, government conspiracies and first contact, good and harmful . That governments around the world are keeping the existence of aliens a secret has, itself, become a meme within a meme. I don't know if the disclosure of first content would be as much of a culture ending event as is often assumed.
It's not aliens, though. It's never aliens.
If they exist, we are at their mercy.