I'm super new to this Stack and even newer to physics. To be honest, I am currently reading an introductory book on physics called "Seven brief lessons of physics" by Carlo Rovelli (the very first book on physics that I've ever read) and I'm finding extremely interesting. As you can imagine I have many questions, but one in particular.

In the chapter on particles, he explains briefly that there are "Elementary particles", and from what I understand they are the ones that compose all things. He then mentions that in some galaxies, astronomers observe a halo of matter, but that it is unknown what these halos are made of. He says it is not elementary particles. Forgive me if my question is dumb (which it probably is); but if everything is made of elementary particles, why can't these halos be made of elementary particles as well? In other words, how do they know they are not made of elementary particles?

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    $\begingroup$ Could you add a little bit more context? Is Carlo talking about the dark matter? It sounds like he is, but I want to make sure before providing an answer. $\endgroup$ – enumaris Sep 25 '18 at 16:27
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    $\begingroup$ Possible duplicate of Why isn't dark matter just ordinary matter? $\endgroup$ – sammy gerbil Sep 25 '18 at 16:36
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    $\begingroup$ See also How do we know Dark Matter isn't simply Neutrinos?, Why can't dark matter be black holes?, What exactly is 'Dark Matter'? and similar questions provided by clicking on the Dark Matter tag. $\endgroup$ – sammy gerbil Sep 25 '18 at 16:46
  • $\begingroup$ @enumaris yes, indeed he talks about dark matter later in that chapter. I will read the other replies above. Thanks just the same :) $\endgroup$ – Paula Sep 25 '18 at 17:26
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    $\begingroup$ I don't think this is exactly a duplicate. The OP isn't asking how we know that dark matter isn't composed of the known Standard Model particles; s/he's asking what it would even mean for dark matter to not be comprised of elementary particles at all. His/her question hinges on a misinterpretation of Rovelli's claims, but an understandable one which other people might also be confused about. $\endgroup$ – tparker Sep 25 '18 at 21:55