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The mean mass densities of cosmic dark matter is larger than that of baryonicmatter by a factor of about 5 in the $\Lambda$CDM universe. Therefore, thegravity on large scales should be dominant by the distribution of dark matterin the universe. However, a series of observations incontrovertibly show thatthe velocity and density fields of baryonic matter are decoupling fromunderlying dark matter field. This paper shows our attemps to unveil thephysics behind this puzzle. In linear approximation, the dynamics of the baryonfluid is completely governed by the gravity of the dark matter. Consequently,the mass density field of baryon matter $\rho_b({\bf r},t)$ will beproportional to that of dark matter $\rho_{\rm dm}({\bf r},t)$, even thoughthey are different from each other initially. In weak and moderate nonlinearregime, the dynamics of the baryon fluid can be sketched by Burgers equation. Abasic feature of the Burgers dynamics is to yield shocks. When the Reynoldsnumber is large, the Burgers fluid will be in the state of Burgers turbulence,which consists of shocks and complex structures. On the other hand, thecollisionless dark matter may not show such shock, but a multivalued velocityfield. Therefore, the weak and moderate nonlinear evolution leads to theIGM-dark matter deviation. Yet, the velocity field of Burgers fluid is stillirrotational, as gravity is curl-free. In fully nonlinear regime, the vorticityof velocity field developed, and the cosmic baryonic fluid will no longer bepotential, as the dynamics of vorticity is independent of gravity and can beself maintained by the nonlinearity of hydrodynamics. In this case, the cosmicbaryon fluid is in the state of fully developed turbulence, which isstatistically and dynamically decoupling from dark matter. This scenarioprovides a mechanism of cohenent explanation of observations.

Statistical and dynamical decoupling of the IGM from Dark Matter