Ok, well that didn't work
Do you have a light source somewhere nearby that might also be contributing to the gradient? Also it looks as though your darks were taken at a different temperature than your lights. See how you have black dots in some of your stars. That could be why that happened. When do you take your darks in relation to your lights and what sort of temperature difference would there be? For every 6 degree drop in temperature you will halve your dark current, though I'm not sure if that includes amplifier noise.