Antiparticles
Every particle in the Standard Model has an antimatter counterpart with the opposite electric charge to their corresponding particles. The electron, for example, has an electric charge of -1 and the antielectron, or positron, has a charge of +1. When an electron and positron collide, their charges cancel out and the particles annihilate in a burst of radiation (photons). However, in the case of neutrinos, it has been suggested that matter and antimatter are the same, that is, that neutrinos are their own antiparticles. This in turn may potentially explain why there is more matter than antimatter and how matter came to dominate the universe in the universe [1].
CP violation - the case of the missing antimatter
In the early universe, the amount of matter (particles) and anti-matter (anti-particles) was the same, but during the universe’s very early evolution the matter and antimatter baryons mutually annihilated into photons, leaving behind a very tiny proportion of matter: for every billion particles of antimatter and matter, one particle of matter was left behind.
According to the principles governing CP symmetry [2], there should be parity of charge between particles and antiparticles, that is a negatively charged electron moving forward in time equates with a positively charged antielectron (positron) moving backwards in time. At the same time, this manoeuvre should also produce a parity transformation: W bosons will only interact with left-handed (clockwise-spinning) particles and right handed (anticloclwise-spinning) antiparticles. So under the Standard Model, exchanging particles for antiparticles is the equivalent of swapping their charge and parity. This is what is meant by CP symmetry.
But if the universe we can see consists only of matter, where did all the antimatter go? Something must have occurred which caused this symmetry to be violated, and under CP violation the decays of particles proceed slightly differently than decays of anti-particles, and one example is in the case of quark mixing [3]. In 1964, James Cronin and Val Fitch discovered that the decay processes of neutral kaons produced pions which were not consistent with CP symmetry. [4] (For this discovery Cronin and Fitch won the Nobel Prize in 1980). Since 1999 further research has been underteken by means of precise measurements with B mesons in an effort to understand the underlying principles. [5]
In the case of kaons which have decayed either into a positive pion (plus negative electron and electron-neutrino) or to a negative pion (plus positive antielectron and electron-neutrino) the decay rate in the former case happens slightly (around 0.3%) more often than in the case of the latter, significant, yes, but nowhere near enough to explain the matter-dominated universe we live in. All we can say is that CP violation is a necessary condition for the matter dominated universe we know and love, but as yet we don’t know the reason why. Neutrinos may have something to do with it, but since they are so small and hard to measure, we cannot yet say.
[1] Source: Martin Hirsch et al, “Ghostly beacons of new physics”, Scientific American, Special Collector’s Edition, August 2013, 20-27. This double beta decay hypothesis which forms the basis for this idea is considered later. Neutrinos are considered in more detail on the page The path to the standard model
[2] Unless otherwise stated, the material in these paragraphs comprises an edited summary of Gavin Hesketh's The Particle Zoo - The search for the fundamental nature of reality, Quercus, Hachette, London, 2016, 216-219.
[3] The phenomenon of quark and neutrino 'mixing' is also considered on the page The path to the standard model
[4] See also Tibor Molnar's "Encyclopaedic Dictionary of Quantum Mechanics" in A Layperson's Guide to the World of the Very Small under "CP conservation, CP Variation".
[5] B. Golob, Physics in Ljubljana Summer School, Ljubljana, July 2011: http://www.fmf.uni-lj.si/storage/20006/Golob.pdf