The particles are virtually identical in every way, except for two properties, one of which is their electrical charge. An electron carries a negative electrical charge, while the positron carries a positive charge. A proton carries a positive charge, while an antiproton carries a negative charge.
Positrons and antiprotons form in a range of processes in the cosmos. For instance, positrons form as a byproduct of collisions between cosmic rays and matter in clouds of dust and gas between stars.
But they trend to vanish quickly; when matter and antimatter meet, they annihilate each other in a sudden release of energy.
This has posed a cosmological conundrum. Current theories hold that when the universe was in its infancy, conditions at the time should have generated matter and antimatter in equal amounts. The inability of matter and antimatter to survive each other should have led to a universe with only a bit of each left as the universe expanded. Yet today's universe holds far more matter than antimatter.
"For reasons no one yet understands, nature ruled out antimatter," says Jeffery Hangst, a physicist at Denmark's Aarhus University as well as a member of the research team for the ongoing project, known by its acronym ALPHA.
In studying antihydrogen in the lab and comparing it with the heavily studied hydrogen atom, physicists hope to pin down the reasons for today's matter-antimatter mismatch.