Can two of the biggest question in physics be solved with just one theory?

Two of the biggest mysteries in physics - what dark matter is made of and why matter dominates over antimatter - might be solved in one go

Different species can have common ancestors. Perhaps different particles do, too.

Hooman Davoudiasl of the Brookhaven National Laboratory in Upton, New York, and colleagues suggest that ordinary matter and dark matter are descendants of the same, heavy "ancestral" particle, for now named "particle X" (Physical Review Letters, DOI: 10.1103/PhysRevLett.105.211304).

Particle X would have had a brief life in the early universe, they say, at which point it decayed into the building blocks of visible atoms and their antimatter counterparts, as well as a zoo of dark matter particles and their antimatter counterparts.

This process of "hylogenesis", named after the Greek word for primordial matter, may seem far-fetched because of its complexity, but it has the advantage of solving the apparent antimatter-matter imbalance. In hylogenesis, dark matter has a property such that the asymmetry between its matter and antimatter particles is able to balance the asymmetry that exists in the visible realm. What's more hylogenesis can also account for the relative abundances of dark matter and ordinary matter.

Unlike WIMPs, the leading dark matter candidates that come in only one variety, hylogenesis calls for two different breeds of dark matter, each with their own antiparticles. Both are too light to match up with the tentative signatures of dark matter seen in experiments today. Such signatures also suggest a particle lighter than a WIMP.

The two very light dark matter particles would be capable of colliding with and destroying protons. The telltale signature of these interactions could be seen in a re-analysis of data collected by the neutrino-hunting Super-Kamiokande experiment in Hida, Japan, which is already hunting for evidence of proton decay.

When a detective finds that two seemingly separate crimes point back to the same suspect, solving each becomes easier. A similar boost could await cosmologists, who are asking whether two of the biggest mysteries in physics - what dark matter is made of and why there was an excess of matter over antimatter in the early universe - have a common origin.

The prospect of solving the two mysteries in one go is not new, but thanks to a batch of fresh models put forward in the last few months, the idea now seems "like it's not an exotic possibility", says Scott Watson of Syracuse University in New York, who helped organise a meeting at the University of Michigan in Ann Arbor last month dedicated to alternative dark matter theories. "It's actually something that we might expect."

Many of these new theories stem from a recent shift in our understanding of dark matter, the substance needed to create enough gravity to stop spinning galaxies from flying apart. The stuff is thought to make up about 85 per cent of the matter in our universe, making it five times more abundant than visible matter. That may seem a high proportion, but physicists want to know why it isn't even greater, and what dark matter actually is.

Countless candidates have been proposed, but weakly interacting massive particles, or WIMPs, hold a special place. These crop up in models like supersymmetry - a popular extension to the standard model of particle physics - yet can also explain the abundance of dark matter thought to exist today. This coincidence is dubbed the "WIMP miracle", and is one reason why WIMPs, until recently, have reigned supreme.

Now that view is changing. "A lot of the model building we've been doing has been based on the observation of a coincidence," says Matthew Buckley at the Fermi National Accelerator Laboratory in Batavia, Illinois. It's time to consider "whether we're being too restrictive; whether we have a one-track mind", he says.

Indeed, experiments designed to detect dark matter have recently hinted that the stuff may not fit the WIMP profile. The underground DAMA experiment in Gran Sasso, Italy, and the Coherent Germanium Neutrino Technology (CoGeNT) experiment in Minnesota have turned up evidence that the dark matter particle is lighter than the mass range favoured for WIMPs. Whatis more, excess gamma rays emanating from the Milky Way were attributed to lightweight dark matter last month.

At the same time, theorists are considering the possibility of alternative dark matter candidates, in case they pop up at the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland. It is within these more exotic dark matter possibilities that a theme is emerging: a range of particles and processes that could all also explain a second mystery.

In the minutes after the big bang, there were a billion-and-one particles of matter for every billion particles of antimatter. The two annihilated, leaving a slight excess of matter, from which all the matter we see today is descended. But the nature of the process that gave rise to slightly more matter than antimatter is unknown.

The new dark matter models (see right) offer explanations for this - as well as explaining the relative abundances of dark matter and visible matter, and what dark matter might be made of. In some models, an asymmetry in the relative components of antimatter and matter within visible matter is transferred to dark matter or vice versa. In others, an imbalance between dark matter particles and antiparticles are created by the same processes and at the same time as visible matter and its antiparticles.

Compared with earlier attempts to link the antimatter and dark matter mysteries, these new theories "are a little closer to our ideas about particle physics now", says Watson. "The ideas are more developed." It's still early days, however. "A lot of these models will turn out to be wrong," cautions Buckley.

The hope is that they can help inform the hunt at the LHC. If WIMPs remain elusive, this new band of particles and processes may also help to shape the next generation of experiments.

Dark materials and lighter matters

Why did the universe end up with more matter than antimatter? Jessie Shelton of Yale University and Kathryn Zurek at the University of Michigan in Ann Arbor say the imbalance could have begun in dark matter particles, which then transmitted the imbalance to visible matter.

In their "darkogenesis" scenario, they suggest that when the hot, early universe cooled, it wasn't smooth but uneven and bubbly - at least in the "dark sector". This led to an imbalance between the number of dark matter particles and their antiparticles. Pretty quickly, "messenger fields" or other processes could have transmitted this asymmetry to the "visible sector"(arxiv.org/abs/1008.1997).

Many models assume dark matter particles must be similar in mass to protons, the main building block of visible matter. Matthew Buckley of the Fermi National Accelerator Laboratory in Batavia, Illinois, and Lisa Randall of Harvard University say this doesn't have to be case.

In their class of models, called Xogenesis (for the monicker "X", which is often given to dark matter particles), the relative abundances of dark and visible matter, and the apparent preponderance of matter over antimatter, arose via dark particles that are much heavier than protons (arxiv.org/abs/1009.0270).

They assume dark matter was more abundant than dark antimatter in the early universe: this imbalance was then passed to ordinary matter.

Unlike models in which an imbalance of matter and antimatter in the dark realm is transmitted to the visible realm or vice versa, in aidnogenesis - from the ancient Greek "aidno" for "dark" - the two asymmetries are created almost simultaneously.

The process began with an imbalance of leptons, a class of visible matter that includes electrons and their antiparticles. Mattias Blennow of the Max-Planck Institute for Physics in Munich, Germany, and colleagues, reckon this then spread to other visible matter and to dark matter. The dark matter particle that best fits this scenario would have a mass of roughly 6 GeV, which matches tentative signatures of dark matter from two detectors (arxiv.org/abs/1009.3159).

The Dark Side Of Antimatter