All particles of ordinary matter (electrons, protons, neutrons) have anti-matter partners that appear identical in all respects (e.g. mass, spin) except that they have the opposite electric charge. We believe that in the Big Bang equal quantities of matter and antimatter were created. The fact that the universe now contains matter and not anti-matter is known as the matter-anti-matter asymmetry. Understanding how this asymmetry was produced is a major goal in particle physics and astrophysics.