The moon doesn’t rotate in the same way that the Earth does. We’re always only seeing one side of the moon, which is why it looks the same from day to day. If you’ve ever heard of “the man in the moon” or that the moon has features that resemble a face, it’s because from our viewpoint the moon doesn’t seem to change and it always has the same side facing the Earth. All that changes is how it waxes and wanes, full moon/crescent moon/new moon and so on.
The dark side is just the back half of the moon that we don’t see because the moon doesn’t rotate. There’s probably not much there to interest NASA. Also, any mission that goes to the dark side will be to be hardened against the low temperature on that side.
Since it gets no sunlight from our home star Sol, it’s really really cold there. We also have no way of naturally communicating with anything on the dark side of the moon as the presence of the moon blocks comms - any mission would need to include a way to route comms to Earth so we can connect the science data. Plus most science stuff can be done on the light side of the moon just as easily. Hope this helps to explain the NASA situation.
I’ve read that strip before but it never made sense to me until now. I was today years old when I realized the play on words that Calvin is making and it’s really funny. Lol!