The signals that get recorded in a light frame can be modified by different factors.

Pixel Sensitivity Variations - Although differences are small, each pixel is slightly different from another in its sensitivity and quantum efficiency.

Vignetting - Can be caused by actual blockages of the light from the corner and edges of the sensor chip by adapters and focusers that are too small. It can also be present for geometrical reasons due to the optics of the imaging system.

Vignetting is visible as darkened corners in this image whose contrast has been exaggerated to make it more easily visible.

Uneven Illumination - Hot spots can be caused by poor baffling. Reflections can be caused by things such as the pick-off prism in an off-axis guider or filters. Blockage can be caused by the mirror in the DSLR interacting with a converging light cone from the optical system causing one edge of the frame to be dark.

Dust - Tiny pieces of dust on the cover-glass that protects the sensor chip can cause shadows that look like out-of-focus dark spots or blobs in the image.

Dust shadows and uneven illumination can be seen in this image whose contrast has been greatly exaggerated. The dust shadows are visible as dark spots.