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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

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    $\begingroup$ The temperature of (gas in) space between the Earth and the Moon is not 3 K; even the first of your links (which does misleadingly claim that "empty interstellar space" has a temperature of 3 K) doesn't say that. $\endgroup$
    – Peter Erwin
    Mar 31, 2021 at 10:41

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The "dark" 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) "dark" side of Moon. This explains the $100 {\rm K}$ you are citing.

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  • $\begingroup$ Why 100K and not 3K (of space) $\endgroup$
    – Ganesh Kulkarni
    Mar 31, 2021 at 10:11
  • $\begingroup$ During the Lunar day (of 300 hours), the Sun has plenty of time to heat up the surface, and the latter is radiating heat back, e.g. towards astronauts. Anyhow, we mostly talk about surface temperature. $\endgroup$
    – B--rian
    Mar 31, 2021 at 10:32
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    $\begingroup$ Even when not illuminated, the far side may well pick up a few degrees from turbulent solar wind. Black body temp. in its orbit is around -23C. $\endgroup$
    – Wayfaring Stranger
    Mar 31, 2021 at 19:43
  • $\begingroup$ This doesn't explain why the surface of the Moon at the end of the lunar night hasn't cooled down to the ambient temperature of the nearby space. Is the surface being kept warm by residual heat in the Moon's interior? Does the nature of the surface have higher absorption than emission? Or is 100 Kelvin the ambient temperature of space at that distance from the Sun? $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Apr 2, 2021 at 23:55
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    $\begingroup$ This is implied but not stated explicitly in @B--rian’s answer: things take time to cool down. If you take a pan of boiling water and put it in the refrigerator, it doesn’t instantly reach the temperature of the refrigerator. So when the sun sets for any given part of the Moon’s surface, the surface starts cooling down, and will continue to cool for about 14 days, until the Sun rises again for that part of the Moon. That’s not long enough for it to cool down to the ambient temperature of space around it. $\endgroup$
    – Eric Jensen
    Apr 3, 2021 at 14:48
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The regolith absorbs solar radiation and conducts some of it to the subsurface during the day. As the lunar night begins, heat is lost rapidly near the surface. As the surface becomes colder than the subsurface, heat stored in the subsurface flows to the surface, and the surface cooling slows. In other words, the rapid cooling seen in the first part of the night in the Diviner graphs is the near-surface cooling, the subsurface cools slowly because it has the near surface as additional insulation.

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