The observable universe seems to consist of almost nothing but matter. It isn't clear why because models of the early universe generally predict that matter and antimatter were created in exactly equal quantities.
Matter and antimatter can come into existence as particle/antiparticle pairs. These pairs exist only briefly before becoming pure energy. Approximately one particle per billion, however, spontaneously decays from an antiparticle into a particle. The reasons for this slight asymmetry are poorly understood, as antimatter is thought to be identical to matter in every way except its electromagnetic charge. The result of this asymmetry is that, on the scale of the entire universe, matter came to predominate.