One of the experiments at CERN has observed D-mesons 'flipping' between matter and antimatter. Antimatter is identical to normal matter but with opposite charge, spin and other quantum numbers.

Mesons are a type of particle made up of a quark and an antiquark. Quarks are the particles that make up the protons and neutrons found in atomic nuclei, and come in six 'flavours' – known as 'up', 'down', 'strange', 'charm', 'bottom' and 'top'. The D-mesons in the CERN experiment are made up of one charm quark and one charm antiquark.

The physicists have witnessed the D-mesons oscillating between being a normal particle and an antiparticle, a process that has previously been observed in K-mesons (composed of a strange quark and an up or down antiquark) and B-mesons (a bottom antiquark and any of an up, down, strange or charm quark). When this happens, the constituent quark becomes an antiquark and vice-versa, so for example the antimatter partner to the K-meson is made up of a strange antiquark and a 'normal' up or down quark. But in some cases this flip-flopping happens at different rates depending on whether a meson is transforming into an antimeson or the reverse is happening.

Experiments in the 1960s showed that K-mesons are more likely to change from their antiparticles to their normal particles than the other way round, and some observations at Fermilab up to 2010 have suggested that the same is true of B-mesons. This is an example of what is known as CP violation – an exception to the principle that physical laws should be the same for a particle as they are for an antiparticle with its direction reversed. This may help to explain why the universe appears to be made entirely of matter with no antimatter except that which is created in high-energy particle collisions.

One would expect the Big Bang to produce equal amounts of matter and antimatter, and, since the two annihilate one another on contact, this should have led to a universe with no particles, filled only with radiation. This problem can be solved if there exists some process that favours matter over antimatter, leading to the excess that we see today.

Alternative explanations include the possibility that there are regions of the universe made of antimatter – which is thought to be unlikely since any overlap with matter regions would produce easily detectable radiation – and the suggestion that antimatter also exhibits gravitational repulsion, which would keep such regions separate.