The D7000 uses a reflex mirror so you are seeing the image directly.
If you need to see the framing I would recommend getting a LED flashlight to aim at the area you are trying to take a picture so you can frame things. Sadly, this will only work at close distances and only with a tripod mounted camera that is fixed.
Maybe you need a different kind of camera, one which uses a live video image through the eyepiece. Some of the newer cameras now offer this. Then you can increase the brightness of the internal display. But again there are limits here.
The last possiblity is using a nightscope which you can mount your camera to. Here you are taking an image of the night scopes screen not the subject directly.
The problem is your lens iris is locked down to the smallest F value (22 or smaller) which is why the eyepiece image is so dark and you had to alter the ISO to even get an image.
Now the hard part is the problem within the cameras control logic or mechanicals or is the issue within the lens? If you have a second lens see if the problem is also present with it. If not then your lens needs service. If the problem is still present then the body need service.
In either case this is a tricky repair and not something I would attempt to fix on my own.
Things that can cause this is storing your equipment in a hot environment. The lubricants used can spread to the iris blades wetting them so they get gummed up or the mechanical control lever which engages the lens is damaged.