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Considering the broken symmetry after the big bang - what I understand as there being a huge surplus of matter and a lesser presence of anti matter - is it possible that dark matter could be anti-matter?

And thus explain the missing anti-matter we might expect from symmetry?

Is there data or any reliable theories that exclude this possibility?

So far our experience with anti-matter is small scale, right? We really don't have experience with anti-matter in bulk. Or are our theories reliable enough to predict bulk properties?

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  • $\begingroup$ How can something that is observed to annihilate matter be responsible to repel it? $\endgroup$ – QuantumBrick Jun 28 '16 at 20:13
  • $\begingroup$ "Dark" means that astronomers can't see it: They can only infer its presence by theorizing about why the things that they can see are distributed in the way that they are distributed. "Dark" doesn't mean anything else besides that. As knzhou said, there's no reason to think that antimatter would be any darker than our kind of matter. $\endgroup$ – Solomon Slow Jun 28 '16 at 20:25
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No, we know enough of the "bulk properties" of antimatter to rule this out. Antimatter interacts with the electromagnetic field in exactly the same way as regular matter, just with the opposite charge.

Therefore, antimatter should be detectable using most of the techniques we use to detect regular matter in astronomy. This works even if the antimatter is a big, cold, diffuse gas of antihydrogen, because we can detect diffuse hydrogen through its 21 cm emission line. (Not to mention the gamma rays we should see from antimatter annihilating with neighboring matter.)

It is precisely because we have not seen these signals that we know dark matter is not antimatter.

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I think we can pretty safely say this is not the case. The main reason is that we have a pretty good idea of where dark matter is--to some degree, it can be reconstructed from the gravitational influence it has on surrounding matter. The dark matter appears to be distributed evenly throughout the galaxy. Thing is, a galaxy is pretty "dirty," as far as space goes. There's plenty of normal-matter gas and dust around, and so we'd expect it to be annihilating with any anti-matter that was also floating around. Since we don't see this (it'd be a pretty persistent gamma-ray glow in the sky) we conclude that there probably isn't a bunch of anti-matter floating around.

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No way, amount of dark matter is estimated to be 4 times the amount of normal matter. Even if we ignore all other arguments, it still does not restore the symmetry. Dark matter (if there is such a thing), is actually transparent matter (we can not see it) and cold matter (it does not absorb/emit any heat/radiation). That also means that you can not measure its temperature. Pretty weird! It only interacts with gravity! Sound more like properties of empty space rather than that of any kind of matter.

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