1. I think its hilarious that "moon" is a word (cause it exists), and "antimoon" isn't a word (cause its fictional)
The dictionary is not an authority on anything beyond terms and usage. Antimoon is simply not common enough to be put in the dictionary. We have many times the words in our language that we used to have, does that make all the new words fictional? No.
2. I think what you're referring to is the submoon (also not a word...) which pulls down while the moon (exists) pulls up.
All of this.. still leaves me with no explanation of the antimoon.. (still not a word)
In order to ask the question you must have heard it used somewhere in some context. The last time I checked, the antimoon was underneath, but the popular name might have changed. It's always possible that the terminology shifted. If you are asking for the definition of something, it is appropriate to provide the context so that I can distinguish if it is being used as another word shadow object or not.