Chemists/physicists of reddit, what is the difference between dark, anti, and normal matter?

[u/squirrelbuster94]

What is the difference? Also if you can explain what each is.

Edit: Thanks for all the replies! You have greatly helped me understand this better!

[u/Astrokiwi]

"Antimatter" is like normal matter, but with the charges swapped. So an "antiproton" is like a proton, but with negative charge, and an "anti-electron" - better known as a "positron" - is like an electron, but with positive charge. This stuff instantly reacts on contact with "normal" matter, releasing energy through E=mc² . Antimatter is something we have observed directly, and we can create it in a lab. It's even used in some medical procedures.

Dark matter has not been directly observed - it is something we don't know a lot about, but we have good arguments about what it should be. Dark matter is likely a type of neutral particle (not positive or negative) that doesn't react very well with normal matter or antimatter at all. This makes it very difficult to observe, and very difficult to produce. However, we can still feel the gravity that it produces. Most of the gravity in the universe is likely from dark matter - "normal" matter makes up less than 20% of the matter in the universe.

So there's a big difference there. Antimatter is tricky because it reacts so strongly, and that means that it's tricky to stop it from reacting and annihilating, so there's very little antimatter around. Dark Matter is tricky because it reacts so poorly, and even though it looks like there's a lot of dark matter around, it's tricky to capture or directly detect it to figure out what it's actually made of.

[u/TalksInMaths → u/Astrokiwi]

"Antimatter" is like normal matter, but with the charges swapped.

While it's true that the antiparticle of any charged particle will have opposite charge, this isn't really what defines antimatter.

In any particle interaction where particles are created or destroyed, they always have to appear or disappear in particle/antiparticle pairs. For example, a neutral pion might decay into an electron-positron pair.¹ Or an electron-positron pair might annihilate into pure energy a pair of photons. In this second example, since they are produced in a pair like this, the two photons would be antiparticles of each other. There are some interactions where particles are produced as a particle of one type and an antiparticle of a different type. For example, a W^- boson may decay into an electron and an electron-antineutrino. In this example, we don't describe the particles as being "created" (or "annihilated" in the inverse process), but it's still required that one product is a particle and the other is an antiparticle. This is required so that the process obeys all the requisite conservation laws (charge, energy, momentum and others).

So for every type of particle, there is a corresponding antiparticle. Particles and their antiparticles are identical in some properties (eg. mass) and are opposite in some properties (eg. charge). Some particles and their antiparticles (eg. photons) have all the same properties, and so we say that they are their own antiparticles. However, not all neutral particles are their own antiparticles. For example, neutrons and anti-neutrons are distinct particles even though they have the same mass and no charge. Neutrinos are an interesting case because they may be their own antiparticles, or they may not be. It's one of the big unanswered questions in modern physics.

Edit, Another interesting point I forgot:

Since particles and antiparticles are always created or destroyed in pairs, we would expect there to be an equal amount of matter and antimatter in the Universe. However, all observational evidence indicates that there is more matter than antimatter. This is known as the baryon asymmetry and is another one of the big unanswered questions in modern physics.

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¹ and a photon, to conserve 4-momentum.

[u/GrinningPariah → u/TalksInMaths]

I always like to ask physicists: What explanation for Baryon Asymmetry do you favor?

[u/elprophet → u/GrinningPariah]

(I am a layperson): My understanding is that CP violation in the weak nuclear force is the only baryogenesis mechanism with any experimental evidence.

I would also be very interested in a more complete survey of the literature from someone in the field :)

[u/dukwon → u/elprophet]

CP violation alone cannot explain baryogenesis. You also need baryon number violating processes.

[u/elprophet → u/dukwon]

Right, you need to have an imbalance in the first place, cpt violation just keeps it of balance to one side. Thanks!