Smudge in middle of digital pix

I have an HP model 720 3.3 megapixel camera, about 4 or 5 years old. I've
developed what looks like a dark smudge in the middle of all the pictures I
take. It isn't noticeable when the subject of the picture is dark, but when
light colored, like the sky, ocean, white wall, etc., it's quite obvious.
It doesn't change in position. I've cleaned the lens repeatedly, to no

Any thoughts? Do I have bad sensors (or whatever they are called)
somewhere in the camera? Is there a fix for this or is the camera shot?