He goes on to claim that the reason this is not more widely known is that elements within the US power structure are deliberately keeping it "under wraps" due to the scary implications such a weapon would have on nuclear proliferation. Since a red mercury bomb would require no fissile material, it would seemingly be impossible to protect against its widespread proliferation given current arms control methodologies. Instead of trying to do so, they simply claim it doesn't exist, while acknowledging its existence privately.
Not to be confused with Red Matter:
The rare mineral decalithium is processed into red matter that creates a black hole when it comes in contact with nuclear matter. It is used in an attempt to avert an impending natural disaster, and to destroy a planet.