Question

what is the reason behind the existence of anti matter?

Answer 1

That's a philosophical question, which is difficult to answer. Physics can't answer "why" something is, but we can describe the way that it behaves. Antimatter is a particle with an electrical charge that is opposite of it's counterpart particle. That is to say, an anti-proton has an electrical charge that is negative, and a proton has a charge which is positive. Matter is much more prevalent in our universe than antimatter, and because the combination of matter and anti-matter causes the destruction of both, antimatter that is created is short lived.