Hi Pierre,
You don't say if you used the 18-105mm f/3.5 wide open or at what aperture; however, it's important to remember that as an all round lens it must try to do many things and that there are some compromises. Light fall off is one compromise especially wide open or thereabouts. This is quantified in this
Lenstip Review link , which reports a 43% light fall-off at 18mm f/3.5 and 37% at f/4. That's quite significant especially when the image is then stretched to bring out dark detail. Couple that with light pollution from city lights and an apparent local stray light source (yellow glow) near the house at the bottom right and hey presto, you have some reasons for the vignetting and gradients seen in the image.
Best
JA