Why didn't matter and antimatter wipe each other out at the start?

Every time a particle meets its antiparticle twin, both vanish in a burst of energy. The early universe was packed with both matter and antimatter in huge quantities, and by the ordinary rules of physics they should have been created in equal amounts, met, and annihilated each other completely, leaving behind a universe of pure light and nothing else. Obviously that's not what happened: you, the Earth, and every star are made of matter, with barely a trace of antimatter anywhere. Physicists call this unsolved puzzle the baryon asymmetry problem, and it's one of the biggest open questions in physics.

The tilt that saved the universe from total annihilation was tiny: for roughly every 2 billion matched pairs of matter and antimatter particles that destroyed each other in the early universe, there was about one extra particle of matter left over with no antimatter partner to cancel it. Everything we see today, every galaxy, star, and planet, is built from that leftover sliver.

In 1967, the Soviet physicist Andrei Sakharov worked out what any explanation for that leftover sliver would need: three conditions, all satisfied at once, including a difference in how particles and antiparticles behave (physicists call this CP violation) and a universe briefly out of equilibrium. Physicists have since confirmed that particles and antiparticles really do behave slightly differently, a discovery that won a Nobel Prize in 1980, but the effect measured so far is far too small to explain the full-sized imbalance that let matter dominate the universe. Sakharov himself is a striking figure. He had helped develop Soviet thermonuclear weapons, then became a defender of human rights. In 1975 he won the Nobel Peace Prize, not for physics, but for that fight.

No theory yet fully explains why the tilt happened, or why it was exactly the size it was. It remains, honestly, one of physics' great unsolved mysteries.