When we look around at the Universe: at the planets and stars, at the galaxies and clusters of galaxies, and at the gas, dust and plasma populating the space between these dense structures, we find the same signatures everywhere. We see atomic absorption and emission lines, we see matter interacting with other forms of matter, we see star formation and stellar death, collisions, X-rays and so much more. There’s an obvious question that cries out for an explanation: why is there all this stuff, rather than nothing at all? If the laws of physics are symmetric between matter and antimatter, the Universe we see today should be impossible. Yet here we are, and no one knows why.