A hypothetical particle called the axion could solve one of physics' great mysteries: the excess of matter over antimatter, or why we're here at all.

According to the Standard Model of particle physics, when our universe was born, the meeting of matter and antimatter should have annihilated each other. That means that nothing -- no Earth, no sun, no galaxies, no humans -- would exist. But we do.

"There's a clear contradiction with the Standard Model," said University of Michigan physicist and postdoctoral researcher Raymond Co. "Why is the whole universe filled with matter, and very, very little antimatter?"

The Standard Model of particle physics explains three fundamental forces in the universe: electromagnetism, the weak force, and the strong force. Electromagnetism is the force between any particles that have a charge. The weak force causes neutrons to break down, and the strong force explains why subatomic particles such as neutrons and protons hold together.

But there are a few contradictions within the Standard Model, one of them being the imbalance between matter and antimatter. The Standard Model also does not explain the existence of dark matter, nor does it explain an observed property of neutrons.

To solve the neutron problem, physicists in 1977 proposed a hypothetical particle called the axion. Five years later, the axion was found capable of solving the problem of dark matter as well. Now, Co and co-author Keisuke Harigaya, a researcher at the Institute for Advanced Study, are suggesting that the axion may explain yet another problem: the imbalance between matter and antimatter. Their research will be published in Physical Review Letters on March 17.

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