The temperature of the space between Earth and the Moon is around 3 Kelvin ($\approx -270^\circ {\rm C}$). The moon has no atmosphere, therefore the heat convection is not possible from the bright side to the dark side on Moon.
Now, if the temperature of the space around the moon is $3 {\rm K}$, then how the dark side of the moon is having a temperature of $100 {\rm K}$?