In the beginning...Strange, but true: energy, if there's a lot of it, can sometimes turn into particles of matter. Every time you make matter out of energy, you also make a 'mirror image' of it called antimatter. They're almost identical - the main difference is that antimatter has an opposite electric charge.
Just after the Big Bang, there was a lot of energy, and lots of matter and antimatter being made. But for some reason we can't find much antimatter out there in the universe today.