No. Antimatter has everything else reversed from matter, but it has positive mass just like normal matter. So to produce 1 gram of matter at the same time as 1 gram of antimatter, you’d need to expend energy equivalent to 2 grams, not 0 grams.
There is a lot of speculation about hypothetical substances which would have negative mass, but nobody’s ever been able to find or produce any, and if it existed, it’d have some extremely weird properties. The best guess is that they don’t and can’t exist.
Come on, man. You can do this thing all day - imagine whatever shadowy organizations you like, with whatever magical technology you like, with whatever near-Godlike ability to cover it all up that you like. If you do that, there’s not a darned thing we can do to disprove your conspiracy theory - you can always posit “well, what if the Bad Guys have magical ability X?”
But that’s not how you analyze the world in any sort of useful way. You look at the information that’s available - and there is no information out there that suggest antimatter weapons are anywhere even remotely close to being on the horizon. For that matter, there’s no indication that anyone particularly wants an antimatter weapon. For reasons ably described by other others, nukes are already far more than good enough for our purposes.
Well there is one thing antimatter would be great for: if you had a compact containment system, you could use microgram quantities of antimatter for “micronukes”. Picture blowing up a tank with a sniper rifle.
