No, it's not, but the fear that if we don't add all that back story readers won't get our stories is something we don't need to sweat over. Readers are pretty smart, and as long as you're keeping them entertained, they'll ride along with you even if they don't know exactly how or why something is the way it is.
And here's a secret:
They don't want to know everything right away. Tell them too much too fast, and there's nothing for them to discover as the story unfolds. That equals a boring story, and who wants that? Readers want to be surprised. They want to wonder why your protag is scared of bright sunlight (when he's clearly not a vampire of course). This wonder will help hook them and make them want to see what happens next.
A handy rule of thumb for dealing with back story is to ignore it during the first draft. Stick to the story at hand, and after it's done, go back and read your scenes. Ask yourself if they make sense as is, or if more information is needed to understand what's going on in that scene. This is key. It might be good information, or important information, but if it doesn't help that scene, save it for later. You don't have to tell it all at once.
Another good way to deal with back story is to background it. You might want to tell readers about the terrible past of your protag, how he spent nine years underground in a Boramese prison. Instead of blurting it out (or worse, shoving in it somewhere it doesn't really fit), think of the things that might have affected your protag because of that experience, and how that might affect his behavior in the scene. Is he extra sensitive to the light? Claustrophobic? Very good at getting around when he can't see well? By backgrounding your back story, you can flesh out your characters and show their history without stopping the story.
And you give yourself a great excuse to mention the back story in context.