A new hypothetical particle could solve two cosmic mysteries at once: what dark matter is made of, and why there's enough matter for us to exist at all.

"We know you have to have these two ingredients to the universe, both atoms and dark matter," said physicist Kris Sigurdson of the University of British Columbia, coauthor of a paper describing the new particle. "Since you know you need those ingredients anyway, it seems like a natural thing to try to explain them from the same mechanism."

Cosmologists think the same amount of matter and antimatter should have been created in the Big Bang, and particles and antiparticles immediately started colliding and extinguishing each other. But the fact that stars, planets and physicists exist now is proof that that's not what happened.

"If matter and antimatter were created in equal amounts in the early universe, they would all have annihilated [each other]," said theoretical physicist Sean Tulin of the Canadian physics institute TRIUMF. "There has to be some asymmetry that was left over."

Together with physicists Hooman Davoudiasl at Brookhaven National Lab and David Morrissey of TRIUMF, Tulin and Sigurdson suggest a way to solve the problem of missing antimatter: Hide it away as dark matter. The details are published in the Nov. 19 Physical Review Letters.

"If our theory is right, it would tell you what dark matter is," Tulin said.

Most of what we know about dark matter is that it is mysterious stuff that makes up a quarter of the energy density of the universe, but refuses to interact with regular matter except through gravity.

The most popular candidate for dark matter is a theoretical weakly interacting massive particle, or WIMP, that connects only with the weak nuclear force and gravity, making it undetectable by eyes, radios and telescopes at all wavelengths. Based on current theories, WIMPs are expected to be about 100 times as massive as a proton, and to be their own antiparticle – whenever two WIMPs meet up in space, they annihilate each other.

The new theoretical particle "is completely different from the WIMP idea," Tulin said. The proposed particle, named simply "X," has a separate antiparticle called "anti-X." Equal amounts of X and anti-X were created in the Big Bang, and then decayed to lighter particles. Each X decayed into either a neutron or two dark-matter particles, called Y and Φ. Every anti-X converted to an anti-neutron or some anti-dark matter.

But the hypothetical X particle would rather decay into ordinary matter than dark matter, so it produced more neutrons than dark matter. Anti-X preferred decaying into anti-dark matter, and so produced more of it.

After all the particles and anti-particles that could find each other collided and eliminated each other, the universe was left with some extra neutrons and a corresponding number of extra anti-dark matter particles.

"The protons and neutrons can't annihilate completely with their antiparticles, because there's not enough to annihilate with," Tulin said. "The same story happens in the hidden sector as well.... Some dark matter can't annihilate with anything. So you're left with some extra dark matter in the universe."

Conveniently, this picture could explain another particle-physics puzzle: why there is only five times more dark matter than regular matter in the universe. To physicists, five is a really small number. If dark matter and regular matter didn't spring from similar origins, there's no reason why there should be roughly the same amount of both of them.

But in the new model, there should be the same absolute number of regular-matter particles and dark-matter particles left after all the particles that can destroy each other are gone. If the dark-matter particles each have a mass between two and three times the proton's mass, then the universe ends up with five times more dark matter than regular matter.

"That's why the light stuff, the visible matter that we all know and love and are used to, is in exact balance with the excess in the dark matter," Sigurdson said. He compares the balance to a yin-yang: "You end up with a little bit more matter and a little bit more antimatter, but they're in exact compensation with each other."

The signatures of this new form of dark matter could be detected by existing experiments. In this model, dark matter doesn't interact with regular matter very often – but it can happen. A dark-matter particle can sometimes smack into a proton or a neutron and destroy it, creating a signature similar to a proton decaying.

Proton decay isn't allowed by the standard model of particle physics, but some theories that go beyond the standard model allow it. An enormous underground tank of water in Japan, called SuperKamiokande, was designed to look for the decaying protons, but has so far found nothing. If physicists at SuperKamiokande went back through their data and looked at slightly different energies, they may be able to find traces of dark matter.

"It's a pretty novel idea," said astroparticle physicist Subir Sarkar of the University of Oxford, who has suggested detecting a different possible form of dark matter by observing its buildup in the sun. The signature of dark matter destroying protons "can be easily tested by the even bigger proposed underground detectors" planned to be built somewhere in Europe.

"This is only the beginning," Sigurdson said. "There's other puzzles out there in particle physics, and we'd like to connect as many of those as possible."

Image: Physicists paddle around the Super Kamiokande detector in a rubber raft as it fills with water. The detector was designed to hunt neutrinos and decaying protons, but could catch the signatures of Particle X. Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research),The University of Tokyo.