The Biggest Mystery in Physics

If the Big Bang created equal amounts of matter and antimatter (and our best theories say it should have), they should have annihilated each other completely, leaving a universe of nothing but energy. Yet here we are — a universe made almost entirely of matter. Where did the antimatter go? This is the baryon asymmetry problem, and no one has solved it. Somehow, for every billion pairs that annihilated, a single extra matter particle survived. Everything we see is built from that tiny leftover.