Antimatter is the mirror opposite of ordinary matter. Every particle has a partner with the same mass but an opposite charge. Electrons have positrons, protons have antiprotons, and when the two meet, they destroy each other in a burst of pure energy.

That makes antimatter fascinating for science and medicine. Positrons are already used in PET scans to image the body. But it is also unbelievably expensive. Because it must be created in particle accelerators one particle at a time, one gram of antimatter has been valued at more than $60 trillion. By comparison, gold, diamonds, and even rare radioactive elements look cheap.

So far, scientists have only ever produced a few nanograms of antimatter, just enough for experiments. 

A nanogram is one billionth of a gram, smaller than a speck of dust. A full gram is far out of reach, but if it could be made and safely stored, it could power starships or completely change the way we generate energy.