Dark shadow on part of iPhone screen?

I dropped my iPhone and now the right half of the screen looks like it's covered in a shadow. It's subtle and I can still see everything but there's clearly a dark patch. I turned it off and on again and the shadow is still there. It has a thin case and a screen protector that's supposed to be impact resistant, and I've dropped it many times before without any problems. Is this permanent damage? Is there any way I can fix it?

No case makes your phone totally impact resistant. Dropping it in just the right way can cause internal damage, even with the case. In your situation, there's absolutely nothing you can do yourself to fix it. You should take it to a phone repair shop and get an estimate as to the repair cost. Then you'll have to decide to keep it as it is, or do the repairs, or get a new phone.