A team of scientists in Hungary recently published a paper that hints at the existence of a previously unknown subatomic particle. The team first reported finding traces of the particle in 2016, and they now report more traces in a different experiment.

If the results are confirmed, the so-called X17 particle could help to explain dark matter, the mysterious substance scientists believe accounts for more than 80% of the mass in the universe. It may be the carrier of a “fifth force” beyond the four accounted for in the standard model of physics (gravity, electromagnetism, the weak nuclear force and the strong nuclear force).

So where did the antimatter go?

This question is one of the biggest mysteries of modern science, and the answer is unknown. Something happened in the earliest moments of the universe to make the antimatter disappear. From our best current measurements of the primordial radiation of the Big Bang (called the cosmic microwave background radiation, or CMB), something tilted the scales in favor of matter, with the ratio of for every three billion antimatter particles, there were three billion and one matter particles. The two sets of three billions cancelled and made the CMB, and the remaining tiny amount of matter went on to form the stars and galaxies that we see in our telescopes today. For this to happen, some physical process had to favor matter over antimatter.

While Einstein’s theory says that matter and antimatter should exist in exactly equal quantities, in 1964, researchers found that a class of subatomic particles called quarks slightly favor matter over antimatter. Quarks are found inside the protons and neutrons at the center of atoms. While this was an important observation, the differences between matter and antimatter quarks were too small to explain the dominance of matter we see in the universe.