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| Baryon Asymmetry |
 Title text: Wait, what do you mean, 'dark matter'? It's not dark, it interacts with high-energy gamma rays ... right? Oh jeez, did I forget to make it interact? |
Explanation
In this comic, Cueball talks with the creator of the universe (previously seen in another comic recently). Cueball wants to know why the universe has baryon asymmetry. The observable universe contains much more matter than antimatter. Current physical theories imply that matter and antimatter should have been created in roughly equal amounts. The fact that antimatter is rare is very fortunate for humans and other objects made of matter, given that matter/antimatter reactions destroy both substances with a violent release of energy.
As it turns out though, the creator of the universe, far from being wise and all-knowing, simply forgot to add a roughly equal amount of antimatter. They then, realising their mistake, proceed to add the antimatter that they originally forgot, only to create a massive explosion as large quantities of the universe experience annihilation.
The creator seems to be very oblivious to the workings of the universe, adding in the antimatter without considering the widespread impact it would have on the universe. This is strange, given they created the universe, and would surely know it's workings. An alternate explanation could be that the sphere is a hidden Black Hat, and knew about the impact yet did it anyway, simply not caring about destroying the universe they created.
The title text resolves another physics mystery in a similarly unexpected way. Dark matter mysteriously interacts with gravitation, but not electromagnetic energy. The creator explains that dark matter was supposed to interact with high-energy gamma rays, but they forgot to add that property to dark matter. The Wikipedia articles for antimatter and dark matter start with clarifications that antimatter and dark matter are not the same thing, and that dark matter is different from dark energy.
The creator of the universe in this comic might be the same floating sphere featured in 3085: About 20 Pounds. Talking to a floating sphere is becoming a returning subject in xkcd.
Transcript
[Cueball is floating in a void. He is speaking with the creator of the universe, who appears as a black sun symbol.]
Cueball: So you're the creator of the universe?
Creator of the Universe: That's me! So, got any questions?
[Cueball is putting his hand to his face as he floats upright.]
Cueball: Oh man, so many. What's the reason for baryon asymmetry? Why is most of the universe matter and not antimatter?
[Cueball's hand is back down he resumes his previous position.]
Creator of the Universe: Crap. Did I forget the antimatter? One sec, let me just…
[Many explosions fill the panel, including a large explosion in the centre.]
BOOM
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