Why temperature of dark side of moon is not 3 Kelvin 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 $100 {\rm K}$?
References

https://sciencing.com/temperatures-outer-space-around-earth-20254.html
https://www.space.com/14725-moon-temperature-lunar-days-night.html
The &quot;dark&quot; side of the Moon is only truly dark during full Moon. Everywhere on the Moon there is day and night as well. The dark side of the Moon is called like that because we do not see it from Earth, since the Moon shows us always the same side due to tidal locking - not because it is always dark there.
In other words: During (lunar)  day, the Sun heats up the surface of the Moon and it gets pretty warm, even on the (not so) &quot;dark&quot; side of Moon. This explains the $100 {\rm K}$ you are citing.