All materials in the Universe are composed of a fundamental subatomic particle. In the early days of the Universe, matter emerged in the form of these fundamental particles. At those times, these particles had their opposite ones. The first particles, which are equal in quantity (make symmetry), destroy each other with their opposites, and the 'matter' that remains today is the barionic substance. These particles, which are the opposite of the material, are called antimatter.

By D. D. Dixon,Public domain, via Wikimedia CommonsAntimatter Cloud/ Antimadde Bulutu

Think of an electron. Anti-substance, the positron. In any case, it is the same as the electron. The mass is also the same as the electron. But the only difference is the electric charge. The electron charge is negative (-) while the positron charge is positive (+). We can think of it for all electrically charged particles. However, when these materials come together in a certain way, they destroy each other and emit light in the form of a gamma radiation.