As an unpublished writer, you absolutely should not submit anything less than a complete novel.
A few quotes to this effect:
You have to have a finished novel.
There are no exceptions to this.
The first step for writing a query letter is to finish the novel.
-- Query Shark
When you send your query, do not send an unfinished manuscript. If you’re writing fiction, agent will want to see a completed manuscript if they are interested in the project. They don’t want to wait for you to finish.
-- Diamond in the Slush
Q: I’m currently writing my first novel and am about halfway finished, but I have completed an outline and synopsis. Can I start querying agents now?
A: Being a first-time novelist, it’s important to finish your book before sending out any query letters. Without a track record, you have no proof that you can finish a novel. And the number of folks who finish writing novels is dramatically smaller than the number of folks who start them (I’m certainly guilty of having three unfinished novels taking up valuable space in my underwear drawer.)
Agents are inundated with submissions and they don’t want to deal with unfinished work. From their perspective, it makes no sense to waste time getting excited about a story that may never get completed (or may not fulfill the promise made in a query letter).
-- Writer's Digest
An additional note here is that you speak of submitting to publishers. I'd strongly suggest you also look into submitting to literary agents. Getting an agent opens a lot of doors for you, and puts somebody else, hopefully a pro, on the task of actually selling your book. And many publishers don't even consider un-agented submissions. So, if you're not familiar with the concept or the potential benefits, definitely familiarize yourself with how agents work and how to find one.
But as you can see, you absolutely need to complete your book before submitting to basically anybody, whether agent or publisher.
But Consider Submitting Your Short Story
It sounds like you have a strong SF or fantasy story on your hands -- and that's something that absolutely has a market in its own right. You can submit it to magazines -- and if you publish it, that will be a solid writing credit to your name. That'll do you a lot of good on submissions for novels as well!