Why doesn't the dust on my cameras sensor show up in pictures?

I have a Nikon D3000 and noticed that when I look in the viewfinder I see a piece of dust. But when I take a picture it doesn't show up. Why is this and is it necessary to have it cleaned?

Its way beyond the focal point of the lens but don't let dust build up as that will affect the amount of light entering.

Any dust you see will be in the eyepiece so it won't be seen by the sensor when the photo is taken.

You only need to have the sensor cleaned if there are lots of fuzzy dark blobs on the image. The way to check the sensor is to take a photo of a blank (and preferably pale) wall with an aperture of f22.

What you will see with a dirty sensor is like in the link

If you see it in the viewfinder, it's not on the sensor. The dust you see would be on the viewfinder screen, the focusing screen, or the mirror.

Maybe you should look at a diagram of the inside of a DSLR sometime…

May be the dust is on the screen and not on the lens. Is it now showing when you zoom the lens to the max?