does antimatter really exist?

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As we all know the atom consist of central nucleus containing containing +vely charged protons and 0 charged neutrons and the orbiting aroung electrons with -ve charge, if the charges are reversed i.e orbiting electron becomes orbiting positron n neuclear particles get -ve charged then we get new type of matter called ANTIMATTER........ but my problem is, is it possible to convert the positron to electron n electron to positron???? and what about there masses...... if we consider that the masses will remain unchanged, then the einstine's equation will fail..... so wats ur opinion??