Matter and antimatter should have destroyed each other in the early universe, but matter survived because of a difference in meson decay:-http://www.nytimes.com/2015/02/11/us/val-fitch-who-discovered-universe-to-be-out-of-balance-is-dead-at-91.html?emc=edit_th_20150211&nl=todaysheadlines&nlid=60788861&_r=0-"In a couple of years they had their answer, and the results shook physics. In about 50 cases, the mesons decayed in a way that was forbidden if nature did not discriminate between matter and antimatter. As Dr. Cronin later explained it in an email, the mesons decayed about 0.3 percent faster into a configuration that included the antiparticle of an electron, called a positron, than they did into one that included an electron.-"But as Dr. Smith pointed out, there was no consensus about the universe at that time. It was only a year later, when radio astronomers discovered incontrovertible evidence that the universe had begun in a Big Bang, that the issue of how matter and antimatter survived mutual suicide became a burning issue.-"In 1967, Andrei Sakharov, the Russian dissident and physicist, put all the pieces together — including the kind of discrepancy that Dr. Fitch and Dr. Cronin had discovered — in a list of conditions that would allow matter to survive the Big Bang.-"Dr. Fitch spent the next decade following up on his famous experiment, and experiments at accelerators in California and Japan have documented the effect to high precision. To the disappointment of scientists, however, the amount of discrepancy so far discovered is not enough, by a factor of more than a billion, to explain the preponderance of matter in the universe today.-
"Following Dr. Fitch and Dr. Cronin's lead, physicists and cosmologists are still searching for the right ingredient to put into Dr. Sakharov's formula.-“There has to be some really ingenious proposal for cosmic data,” Dr. Smith said."

Another candidate for dark matter. We've had gravitinos, WIMPS, and now Axions:- https://www.quantamagazine.org/20160107-arrow-of-time-axions/-"The theory of axions predicts, in a general way, that axions should be very light, very long-lived particles whose interactions with ordinary matter are very feeble. But to compare theory and experiment we need to be quantitative. And here we meet ambiguity, because existing theory does not fix the value of the axion's mass. If we know the axion's mass we can predict all its other properties. But the mass itself can vary over a wide range. (The same basic problem arose for the charmed quark, the Higgs particle, the top quark and several other others. Before each of those particles was discovered, theory predicted all of its properties except for the value of its mass.) It turns out that the strength of the axion's interactions is proportional to its mass. So as the assumed value for axion mass decreases, the axion becomes more elusive.-***-"The experimental search for axions continues on several fronts. Two of the most promising experiments are aimed at detecting the axion fluid. One of them, ADMX (Axion Dark Matter eXperiment) uses specially crafted, ultrasensitive antennas to convert background axions into electromagnetic pulses. The other, CASPEr (Cosmic Axion Spin Precession Experiment) looks for tiny wiggles in the motion of nuclear spins, which would be induced by the axion fluid. Between them, these difficult experiments promise to cover almost the entire range of possible axion masses.-"Do axions exist? We still don't know for sure. Their existence would bring the story of time's reversible arrow to a dramatic, satisfying conclusion, and very possibly solve the riddle of the dark matter, to boot. The game is afoot."