Is An Explanation Really Necessary?
As a follow-up to my last post, The Devil's in the Details, you may also find the article, Worldbuilding: Constructing a SF Universe, by S. Andrew Swann, interesting. One thing I noted was his recommendation that any alterations to the world the readers know be explained. I'm not sure I agree entirely with this. In general, I'd say that if the difference can be shown and is not so extreme or strange that it will confuse readers or throw them out of suspension of disbelief, then you shouldn't explain, unless it's crucial to your story. At the same time, I know that many science fiction novels have a lot of technical information in them and there are many readers for whom this technical detail is very important. So, maybe that's a matter of knowing your audience.