If the universe was made of antimatter then we would probably call antimatter "ordinary matter" and what we now call "matter" would be called "antimatter". What if during baryogenesis there was a huge amount of matter and antimatter that collided and created photons and electrons and that the matter that we see now is only a tiny fraction of the matter that once existed? We are made of "ordinary matter" only because ordinary matter happened to slightly outnumbered antimatter. Even if antimatter slightly outnumbered regular matter, would we be able to tell?