The nonlinear evolution of a system consisting of collisional baryons and collisionless dark matter is generally characterized by strong shocks and discontinuities in the baryon fluid .
The baryons slow down significantly at postshock areas of gravitational strong shocks , which can occur in high overdense as well as low overdense regions .
On the other hand , the shocks do not affect the collapse of the dark matter .
Consequently , the baryon fraction would be nonuniform on large scales .
We studied these phenomena with simulation samples produced by the weighted essentially nonoscillatory ( WENO ) hybrid cosmological hydrodynamic/ N -body code , which is effective at capturing shocks and complex structures with high precision .
We find that the baryon fraction in high mass density regions is lower on average than the cosmic baryon fraction , and many baryons accumulate in the regions with moderate mass density to form a high baryon fraction phase ( HBFP ) .
In dense regions with \rho / \bar { \rho } > 100 , which are the possible hosts for galaxy clusters , the baryon fraction can be lower than the cosmic baryon fraction by about 10 % –20 % at z \simeq 0 .
We also find that at z < 2 , almost all the HBFP gas locates in the regions with mass density 0.5 < \rho / \bar { \rho } < 5 and temperature T > 10 ^ { 5 } K , and conversely , almost all the gas in the areas of 0.5 < \rho / \bar { \rho } < 5 and with temperature T > 10 ^ { 5 } K has high baryon fraction .
Our simulation samples show that about 3 % of the cosmic baryon budget was hidden in the HBFP at redshift z = 3 , while this percentage increases to about 14 % at the present day .
The gas in the HBFP can not be detected either by Ly \alpha forests of QSO absorption spectra or by soft X-ray background .
That is , the HBFP would be missed in the baryon budget given by current observations .