An imperfect mirror

Scientists have wondered whether all the antimatter exists in its own part of the universe, but they've seen no evidence to support that idea.

Instead, they think that some antimatter isn't quite a perfect mirror image of matter - there's an asymmetry. When matter and antimatter collide, they annihilate each other and turn into energy. If antimatter isn't a perfect mirror image of matter, then some of the matter survives the collision. Scientists call this idea of asymmetry 'CP violation'.