Antimatter
Because antimatter can make so much energy, it can be used for a lot of things, such as fuel for going into outer space, or in our cars. The problem is that antimatter is very expensive to make, and is almost as expensive to store, since it cannot touch regular matter. It takes several hundred million dollars to make less than one-millionth of a gram of antimatter. In fact, it is the most expensive and the rarest substance on Earth. Since it is so expensive, this means that antimatter is not practical to use as a weapon or as an energy source, because so little of it is obtainable.