In the story I'm creating, a major weapon type of humanity will be Antimatter based weaponry. This is a fairly new technology, as antimatter production facilities have just reached the size and capacity to realistically produce the stuff in necessary quantities to use. I know that energy-density wise, antimatter is significantly better than anything humanity has produced up to this point, and as such would be more significantly powerful than thermonuclear weaponry. In the modern day, though it has been studied for use in weaponry by the US Air Force, antimatter is not used due to extreme production costs and low production amounts. Aside from production costs, what are the major downsides to using antimatter?