Why is my nikon D80 is taking very dark photos?

Its the middle of the day and I want to take photos inside, lights are on as well, and my camera is taking very dark pictures

Its on Manual
shutter speed 1/80 sec
aperture 4.5
ISO 100
lens Nikon nikkor 18-35mm

What is going on?

Well… Could be it's on manual and you have no clue how to use the build in light meter.
Could be you've accidentally set it to -3EV
Could be a sticky aperture.

Set it on iso 400 and P mode and see what happens

ISO is too low, and open the aperture as far as it will go. Unless your subjects are moving around a lot, you can also reduce shutter speed to 1/50. ISO at least 800, aperture open, reduce shutter.

1) continue to open your aperture wider until your light meter is on 0. What does the aperture affect? The wider the aperture is (or the smaller the number) the more light is let in. Also, your aperture affects your Depth of Field, and the wider your aperture is, the smaller depth of field you have, so more will be out of focus. This could be a cool effect for equine photography! (Just a note: when you zoom in with your lens, your minimum aperture will get a bigger number, so if you zoom in and out a lot, make sure you check your light meter every so often).

2) if this doesn't work (but it should for outdoors in the day time), then keep your aperture wide open, and boost your ISO up. This will make your sensor more sensitive to light, so you will get brighter pictures.

Hope this help: D

Broken

In manual mode your camera will do exactly what it's told to do; it will not try to correct what you are doing at all!

Put the camera in "A"perture mode with the aperture set to f/4.5, and "fix" the ISO at 100. Take a photo and see what shutter speed it uses.

Put the camera in "S"hutter mode and set the shutter speed to 1/100th of a second, keep the ISO "fixed at 100. Take another photo and see what aperture it sets - it may even warn you that it's outside any acceptable range (in fact if you know how to read it it, I'm sure it will do this).

Now, put the camera in to "P"rogram mode again with the ISO "fixed" at 100. You'll see that as you open up the aperture the shutter speed gets faster, and as you close down the aperture the shutter speed gets slower. This mode attempts to provide a balance between the two, while maintaining good exposure.

Think on the above and I hope you'll start to understand what is happening.

Repeat after me, 500 times: 'The camera is not taking the photos, the photographer is taking the photos.'

The camera is doing what you are telling it to do. You are telling it to take dark photos. Learn about exposure. As a start, take the radical and little-used step of reading the camera manual.

1. Set your mode dial to "P"
2. Press the shutter half way
3. Look at the display in the bottom of the viewfinder and see what shutter speed and aperture the camera selected
4. Switch your mode dial to "M"
5. Select the same shutter speed and aperture that the camera selected in "P" mode
6. Take the picture
7. Tell all your friends that you took the photo using manual mode, because you did

It will take dark images with those settings… You'll need to up the ISO to start with… ISO400 will give you two stops more light. Shooting at the wide 18mm end of the lens will help get light into the camera and increasing the aperture size to f/3.5 will all help. You may still need to slow the shutter speed in which case you'll need a tripod to eliminate camera shake. Use the in-built light meter on your camera.

ALTERNATIVELY, flash is your friend… Even on-camera flash will allow you to take shots inside, although the lighting will be somewhat harsh

The ISO is a bit too low.
The aperture needs to be boosted as well. The shutter speed is fine.
Is the lens cap on?

Bump up the ISO…200 or 400… Use fill flash.

It is NOT the camera, It is your lack of knowledge.

You are inside. Your eyes/brain adjust to light levels. Cameras don't until you adjust them.
You may THINK there's a lot of light but if you were to check your light meter you would find out there in fact isn't.

Go buy a book on basic photography and learn about light and exposure.

In this instance you will have to either lower the shutter speed, open the aperture more or raise the iso, or a mixture of all three.