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?