Dark lines on Panorama?

As fhotoace says, it's vignetting. All lenses have this issue, although it varies. When stitching panoramas good software will get rid of the vignetting. The stitching mechanism you're using, which I'm guessing is in-camera, is simplistic. You are better off taking the individual photos and merging (stitching) them in a good program like Photoshop or Hugin.

If you have access to a modern version of Photoshop, use Photoshop's stitcher (File>Automate>Photomerge) to create the panorama and check the remove vignetting box.

If you don't have access to Photoshop, then you can use a free program called Hugin. It's an excellent stitcher, however, it's a wee bit harder to use than Photoshop's stitcher.

I agree it's vignetting caused by the camera. However, you could attempt to fix it in Photoshop. Use the dodge tool, set the exposure to about 20% or less, use a very large soft edged brush. Better than nothing I guess.