Nitty Gritty Details of the Non-Existent

Writing for me is often like combing tangled hair. The strokes that I first take are short, ending abruptly, each such stroke usually being the major part of a sitting's work. The next stroke, at the beginning of the next sitting, usually has me comb through the same hair making sure the knots are gone, before attacking the tangle once more.

In combing the same material I find certain consistencies, for example I always seem to be providing a thought and then notting or norring it. But let us ignore that for now, did not bring about this ramble. Instead it deals with questioning the level of detail to go into during explanations

When I write, I often worry about leaving unanswered questions, something that makes sense based upon my job. But unlike my job, when writing fiction much that I write about is non-existent. At the minimum, my characters and the situations they find themselves in. Further, because I mostly write sci-fi or fantasy, the technology, magic, worlds, universe do not exist and it is in these details that I often get caught.

In my initial writings, I find myself trying to answer all questions; however, upon review it often makes the flow better to delete some explanation. Therefore, I now need to figure out what to dump. For me, that often tends to involve keeping things that are visual in nature, while removing a + b descriptions.

What I was wondering, is how writers determine the level of detail for their stories? And from readers, how much detail do you want?