Now here is a name for an effect that I have observed and been puzzled about!
Alexander’s dark band – – Alexander of Aphrodisias first described the effect with double rainbows in 200 AD and it now carries his name.
Light rays undergoing a single reflection in raindrops form the primary rainbow or brighten the sky inside it. Rays reflected twice are deviated to form the secondary bow or brighten the sky outside.
Raindrops along lines of sight between the two bows cannot send light to your eye and so the sky is darker there.
You can see the effect in this image I posted a year or so ago.
Read more at https://www.atoptics.co.uk/rainbows/adband.htm or https://en.wikipedia.org/wiki/Alexander%27s_band