What is amazing is the way that the latest experiment was done. Scientists created "anti-protonic helium."
Antimatter is extraordinarily difficult to handle . . . because upon coming into contact with ordinary matter (even the air molecules in a room), it immediately annihilates, converting into energy and new particles. . . . antiprotons produced in high-energy collisions are collected and stored in a vacuum pipe arranged in a 190-m-long racetrack shape. The antiprotons are gradually slowed down, before being transported . . . into a helium target to create and study antiprotonic helium atoms.
Normal helium atoms consist of a nucleus with two electrons orbiting around it. In antiprotonic helium, one of these electrons is replaced by an antiproton, which finds itself in an excited orbit some 100 picometres (10^-10 m) from the nucleus. Scientists fire a laser beam onto the atom, and carefully tune its frequency until the antiproton makes a quantum jump from one orbit to another. By comparing this frequency with theoretical calculations, the mass of the antiproton can be determined relative to the electron.
You won't find this element in the periodic table. Indeed, it may never have arisen in nature. But, it was possible and was created. Indeed, since the calculations regarding the forces that govern a negatively charged particle in orbit around an atomic nucleus are know with much greater precision than other mathematical constants, it may ultimately be easier to measure an antiproton's mass than a proton's mass.
Hat Tip to Maju.