The question why the Universe is matter dominated, instead of being made of equal parts matter and antimatter, is still unsolved as of today. One of the conditions required to develop the observed dominance of matter over antimatter is the violation of the Charge-Parity (CP) symmetry. This says that the laws of physics should be the same if viewed upside-down in a mirror (P), with all matter exchanged by antimatter (C). If CP violation occurs in neutrino physics, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos. The international T2K Collaboration recently observed that the electron antineutrino appearance rate is lower than expected from the electron neutrino appearance rate, assuming that CP symmetry is conserved.