Image through nikon f80 dark and cloudy

A few months ago I bought a second hand Nikon f80. All its functions are working since I last tested, although the batteries have died out since then. However, whenever I look through the viewfinder there is a dark and cloudy rectangle which obscures the whole image; around it is a clear outline, and there is also a small black curve across the top (definitely something else). If the camera is pointed at bright light, then this dark patch seems to fill up with white dots. I'm sure it isn't the lens, which I've cleaned, because the image remains the same if I remove the lens and look through the viewfinder. I don't think it is the mirror, which is quite dusty but otherwise looks fine. The focusing screen also looks normal, although there is a small scratch (which I can't see through the viewfinder anyway). I can't think of anything else; so I was wondering if anyone could tell me what it is and how to fix it/how much it would cost to fix, if it can be at all. Thanks,
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