Page "Antiparticle" Paragraph 2

The laws of nature are very nearly symmetrical with respect to particles and antiparticles.

For example, an antiproton and a positron can form an antihydrogen atom, which has almost exactly the same properties as a hydrogen atom.

This leads to the question of why the formation of matter after the Big Bang resulted in a universe consisting almost entirely of matter, rather than being a half-and-half mixture of matter and antimatter.

The discovery of CP violation helped to shed light on this problem by showing that this symmetry, originally thought to be perfect, was only approximate.