Do Antiparticles Exist?
Physicists thought that Paul had a good idea. And remember that physicists are a bit like your school friends: if you tell them your sister is an alien, they wouldn't believe you until you prove it!
So you can imagine what happened when Paul suggested that antiparticles exist: physicists wanted proof and started to look for them! Their idea was to take a bit of energy-dough and try to make particle-antiparticle twins.
Their first success was the creation of an "electron-positron" pair, twins that only require a relatively small amount of energy-dough to make them. Later came pairs of protons and antiprotons, then pairs of neutrons and antineutrons - but only after physicists had learned how to make enough energy-dough!
So Paul was right! antiparticles do exist! Of course, the antiparticles created in a laboratory live for a very short time before they crash into normal particles and annihilate. But nevertheless, they do exist.
This means that when the electrons, protons and neutrons which make our world were created, a lot of their twins (positrons, antiprotons and antineutrons) were also made. So, where are they? Where did they go? Did they become an antiworld?