cosmology - Why doesn't dark matter interact with ordinary matter?

Dark matter doesn't interact electrostatically but I don't understand why it does not collide with ordinary matter . Also can a chunk of dark matter interact with another chunk of dark matter if so how? .Also are there any established experimental protocols for detection of dark matter in a direct manner , that is not by unseen mass present in a galaxy.

Answer

In one sense you raise the question of whether a type of matter or elementary particle can exist without interacting with known matter or particles. Of course gravitation is a form of such interaction, but if that is the only way dark matter (DM) interacts with known matter then it is in a sense "forever dark."

The attempt to detect DM by it collision with Xenon atoms is outlined by EasyPeasy. The assumption has been that DM is most likely interacts by the weak nuclear force, and a condensate of the super-partners of the photon, Higgs particle and the neutral current Z particle called the neutralino is a candidate for DM. If so then a neutralino, or any weak interaction DM particle, a WIMP, would interact with an atom. This would then translate some of the tranverse momentum or energy into another form. The Cryrogenic Dark Matter Search (CDMS) attempts to find such an interaction with a cold crystal. A DM particle will then in principle induce a phonon or quanta of vibration on the lattice.

So far these searches have come up empty. If such searches as well as the LUX-ZEPLIN, a scaled up version of ZEPLIN that has come up null in the search, fail to find anything then we may have to abandon the WIMP hypothesis. The null data so far from the LHC for low mass or low energy supersymmetry means the neutralino (as well as s-quarks etc) may not exist at multi-TeV energy.

The alternative is the axion, which is found in the supergravity multiplet, and is also the carrier of the CP violation for QCD. This has some interesting connections between gravity and QCD in a supersymmetric setting at much higher energy. The axion is a scalar particle that obeys the wave equation $$ (\square~+~m^2)\phi~=~\kappa\vec E\cdot\vec B $$ where this inhomogeneous term term $\kappa\vec E\cdot\vec B$ means the axion can be exchanged with photons. There are searches for this. An axion in the presence of a strong magnetic field can be converted to a photon. The Axion Dark Matter Experiment (ADMX) is an attempt to find such conversions. This will be very difficult for the axion mass that is a measure of the CP violation is expected to be very small.

An interesting paper came out recently on an anomalous decay of $^8Be_4$ that suggests an $17MeV$ particle may be produced. This is argued to be a new type of gauge particle that mediates interactions primarily in the dark matter sector. This may or may not turn out to be how nature works. It does have some connections with axions, and it also makes the point that dark matter might be a whole "zoo" of various types of quantum fields or particles.