Excellent drawing! Looking at it as a fellow artist, I think ti could be problematic at this point to try to go in with a dark background next to the greys and darks although I like the idea of a shadowy moon. My art teacher always tells me not to leave the background til the end but to start working on it early so you don’t get stuck.
If you do go for a dark background, how will you fill it in? Soft pencil shading? Cross-hatching? It does seem somewhat incomplete but I’d be inclined to do another and work the background in while doing the foreground. However, you can always experiment with what you have. (Another thing my art teacher says is, “You must kill your babies.”)