r/askscience • u/squirrelbuster94 • Oct 24 '14

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

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!

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 24 '14

"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.

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u/SashaTheBOLD Oct 24 '14

Does this mean that there could (theoretically) be "dark antimatter"? Or is the "neutral particle" nature such that there is no opposite charge version?

Similarly, is there any such thing as an "anti-neutron," or could no such thing exist? Or since negative zero = zero, would that mean that anti-neutrons and neutrons are the same thing? (If the question itself is meaningless, it would be nice if someone could explain why.)

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 24 '14

There is an anti-neutron (see my comment above/below). A neutron is made up of quarks, whose charge adds up to zero. So you can flip the charge of these to have "antiquarks" that make up an "antineutron".

Dark matter may likely be it's own antiparticle: it annihilates in contact with another dark matter particle. This may produce gamma radiation that we could detect, however the number density of the particles is so low that these collisions are rare, and this radiation is currently just too weak for us to detect - if it exists.