Polarization effect.
Maximum light polarization occurs 90° from a light source. The fish-eye lens is a great tool to demonstrate this. Note that the sky becomes lighter again as you go farther away from the Sun. I love demonstrating this to students who visit the observatory. None of them have ever noticed this effect.
The same holds true for Moonlight. The night sky becomes darker as you move 90° from the Moon. Good to know next time there are meteor showers during a bright Moon!
(This an un-processed JPEG image. Vignetting is almost non-existant at this faster aperture.)

Canon EOS M100
10mm, f16
1/2000s@ISO160

Top and bottom of the fisheye image were cropped out.