Is scattering an interaction? The way I understand scattering in classical physics is: it is underlying electromagnetic interaction plus the exclusive principle. So I had thought it would have no scattering nor interaction between dark matter particles, but it seems not a good guess, as people are all calculating dark matter particles' cross sections. 
So what is under the hook for the scattering between electrically neutral particles? What exactly happening between fundamental particles scattering ? Are scattering consider some interaction? Or scattering is just some aftermath of the Exclusive principle? 
 A: 
Is scattering an interaction?

Yes, scattering is an interaction between quantum mechanical entities. There are four fundamental forces that are involved in scatterings and decays.
The strong  which has a coupling of 1 is responsible for the nuclear forces and the periodic table of elements.
The electromagnetic , which has a coupling of 1/137
The weak which has a coupling of $10^{-6}$
The gravitational which is very much weaker $ 6 x 10^{-39}$
The hypothesis of dark matter particles called WIMPSmeans that these hypothetical particles interact only weakly. Since to see them electromagnetic light is required, their existence is "dark", though they may interact/scatter off each other or decay with the weak force .  These interactions can be computed, as you mention but as no or few photons come out of the interactions, the WIMPS are dark.
A: 
The way I understand scattering in classical physics is: it is underlying electromagnetic interaction plus the exclusive principle.

There's no exclusion principle in classical physics. Classical physics means physics without quantum mechanics.

So I had thought it would have no scattering nor interaction between dark matter particles, but it seems not a good guess, as people are all calculating dark matter particles' cross sections.

They're assuming that dark matter interacts through the weak force.
