Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.
Some of the candidates for where all the missing mass is, is poorly interacting matter
I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?
You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.
But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.
So you might not detect photons in one particular direction, but we should assume that they should be going in all directions. Otherwise that theory is just moving the goal from "where's this energy" to "why is it pointing towards that".
CMB is consistent with, every direction is the same, so you would have to say "photons are going everywhere in this range, but towards that in this segment of the spectrum".
However there's this one experiment that says the answer is `Energy = 42`. `f(x)` is still useful, everything else works.
We can just amend the equation to be: `Energy = Mass + Photons + DarkMatter` where `DarkMatter = -42` and `f(DarkMatter) = 0`.
Or we can keep the E=M+P but change the way f works to be: `f(Photons, X) = Y` where for almost any value `f(Photons, X') = 0` and in this one case `f(Photons, X") = 42`.
To me it's clear that the first approach is less complicated. The first approach raises one question, "why -42?" The second approach raises "why does X?" exist and "why 42?"
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By the way, you are mistaken, we do interact with Dark Matter, that's how we detected it (Bullet Cluster). Gravity does interact with it.
[1] https://www.quora.com/How-does-a-photon-exert-gravitational-...
edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe.
It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.
The empirically observed expansion of the Universe is the motivating evidence for the existence of Dark Energy. Dark Matter, which is motivated (in part) by the inconsistencies in the rotation curves of galaxies is a separate topic. Despite sharing similarities in their names and the fact that they make up the two biggest chunks of energy/mass in the Universe, Dark Energy and Dark Matter are not actually related. The word 'dark' is really just implying that we have not yet observed anything to explain these two phenomenon.
That's how stars are formed.
Absolutely none of this is applicable to dark matter. Dark matter isn't made up of atoms, it doesn't bump into and bounce off of other particles of matter. It doesn't maintain a temperature and pressure the way a gas does. Dark matter interacts extremely weakly. Neutrinos are an example of dark matter, but a type that we know doesn't make up most of the mass of dark matter in the Universe. A neutrino will pass through a chunk of lead a light year thick and then only have a 50/50 chance of being stopped. Dark matter is even more weakly interacting (with ordinary matter and itself). Particles of dark matter are zooming about in orbits around the center of mass of our galaxy. They zip through almost everything they touch without interacting, the exception being black holes, which they simply fall into like everything else, of course. They are like an enormous parade of ghosts that can only interact with other matter through gravitation, meaning orbital dynamics. Because of this they have no way of condensing into forms of higher density like nebulae, stars, or planets.
Imagine a giant ball of yarn larger than our galaxy, except each thread is a flow, a river of huge numbers of ghostly dark matter particles. Except there are many balls of yarn overlayed on top of one another and many flows going through any one point, since they don't interact with each other. Some are traveling around in orbits around the Milky Way in the same direction as our Solar System, some are going the opposite way, some are in orbits at various inclinations to the galactic plane, some are in circular orbits, some are in eccentric orbits and the part of the galaxy where we are is their highest distance from the galactic center, for others it's the closest distance to the galactic center, and so on. All of these flows, this ghostly wind of insubstantial but massive dark matter particles is what together makes up the "dark matter halo" around our galaxy and around typical galaxies. In any given section of the galaxy, say a typical cubic light year, the total amount of dark matter isn't that great, it's vastly lower than the mass of any star that would happen to be there, for example. But the dark matter is everywhere, it flows throughout a region that extends well beyond the edge of the visible galaxy, and it has roughly the same density everywhere, so over huge volumes that mass adds up, and up, and up, and turns out to be, in aggregate, greater than the mass of all of the ordinary matter in our galaxy, by a factor of about 5:1.