Sunlit raindrops anywhere send light back sunwards. It is mostly directed into an intense circle around 42° away from the antisolar point.
There is an infinity of unseen rainbows. Does a rainbow exist if there are no eyes to see it or brains to comprehend?
The primary rainbow that we do see comes from the glints of those raindrops that lie near the surface of a cone. It extends downwards from our eye (one eye because each eye sees a different bow) with its axis towards the antisolar point.
Drops outside the cone glint elsewhere and we only see dark sky - Alexanders dark band.
Drops inside the cone do glint towards us. But faintly. The eye mixes their colours to white. We see sky brighter there than elsewhere