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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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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. –  dmckee May 11 at 15:40
    
Which other problems are you referring to, other than potentially the hierarchy problem (I don't know very much about cosmology)? –  JeffDror May 12 at 15:14
    
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. –  dmckee May 12 at 16:03
    
But how can you explain the relic density of DM if it does not couple to the SM particle? –  Melquíades May 13 at 15:37
    
@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) –  JeffDror May 13 at 16:51

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