People always talk about finding good dark matter (DM) candidates of beyond Standard Model (SM) theories. To be a proper candidate the particles should not decay into the SM particles and could potentially be weakly interacting mass particles (WIMPs), but this doesn't need to be the case. If DM is not composed of WIMPs then can we just add DM through the simplest and most boring way, \begin{equation} \Delta {\cal L} = m_{DM}^2 \phi_{DM} ^\dagger \phi_{DM} \end{equation} to the SM? Is there something wrong with this naive addition (other than it not being predictive)?
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$\begingroup$ Not an expert, but I suppose it could ... only then we'd still be looking for solutions to the several problems other than gravitational rotation curves that people hope the dark sector is going to solve for them. $\endgroup$– dmckee --- ex-moderator kittenCommented May 11, 2014 at 15:40
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$\begingroup$ Which other problems are you referring to, other than potentially the hierarchy problem (I don't know very much about cosmology)? $\endgroup$– JeffDrorCommented May 12, 2014 at 15:14
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$\begingroup$ As I understand it, you can't both solve the structure problem and explain the current overall flatness of the universe with a single dark matter temperature, but you can do these things if you allow the dark matter to cool by some mechanism stronger than pure gravitational interaction. I think that baryon acoustic oscillation is often discussed in this context. $\endgroup$– dmckee --- ex-moderator kittenCommented May 12, 2014 at 16:03
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$\begingroup$ But how can you explain the relic density of DM if it does not couple to the SM particle? $\endgroup$– MelquíadesCommented May 13, 2014 at 15:37
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$\begingroup$ @Melqulades: So is the zero'th order issue with such a proposal that the relic density remains a free parameter with no explanation? (I should mention that I am really out of my element here, so sorry if what I'm saying is nonsense) $\endgroup$– JeffDrorCommented May 13, 2014 at 16:51
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There is nothing wrong with such an assumption. However, proving it will not be that easy. On the other hand, disproving it will be neither... ;)
People are looking for WIMPs because they are possible to detect. As long as we have still theories with such particles that need to be (dis-)proven, we will try to do it.
There are a number of theories, and not all the DM candidates are WIMPs. For example in the Higgs portal approach, there was the idea of a colourd scalar.
What we need to keep in mind is, that luminous matter, described by the Standard Model, is very complicated and has a lot of particles in it. So to assume that dark matter will be only one scalar particle is very naive.
