why flux through outside charge is 0 in gauss law?

 The expression of gauss law states that 

Φ = Qenc. ⁄ ε0

It is interesting to see that this formula says that the flux through a closed surface is only due to the charges enclosed in it.

explanation: Let us consider that we have a spherical surface M and we have a point charge +q outside the surface so here we can see that the flux coming in is equal to the flux going out so the flux coming in can be taken -ve then the flux going out is +ve so adding both of them we get the net flux =0, here it can be noted that the flux through a given point on the surface M is not 0 but adding fluxes of all such points on the surface M is equal to 0 so the net flux on M is 0. 

Thanks for Reading!

-Arvin Gupta