system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Dark matter, dark energy, and antimatter

What are they? What is their nature/ what do they do? Which are theoretical? Examples would help too.
gpt
Antimatter is not theoretical at all. We've created it. It's made from anti-particles, which share the same properties as their "normal" counterparts, except with the opposite charge. So positrons are exactly like electrons, only positively charged, anti-protons are just like protons, only negatively charged, etc. As far as we can tell, an entire universe made of antimatter would be identical to one made of matter. In fact, the only reason we call ours matter and the other stuff antimatter is because matter happens to be what we're made out of. It's the same with positive and negative charges. There's nothing special about one or the other, they're just opposites, so the fact that we named one positive and one negative could have just as easily been the other way around. When matter and antimatter come in contact with each other, they annihilate, meaning they cease to exist as matter/antimatter and are converted to energy. There is no such thing as antienergy.

Dark matter and dark energy are theoretical. The concept of dark matter was created as an explanation to why galaxies are held together. Taking all the matter we can see in our galaxy and others, there doesn't seem to be enough stuff there to explain why gravity holds it all together. So the hypothesis is that there is something we can't detect that has gravity (and therefore by definition is some kind of matter), which we call it dark matter.

Dark energy is created to explain what is causing the rate of expansion of the universe to increase. Again, that behavior indicates that there must be some force that we can't detect which is causing it, so we call it dark energy.