Rainbows form when light is refracted and reflected in droplets of water from rain (or some other source) as shown below. The light emerges at angles of up to 42°, so that the primary rainbow forms a circular halo around the antisolar point, at an angle of 42° from it. For the secondary rainbow, light enters the droplet from below and is internally reflected twice, emerging at angles of 51° or more, thus forming a larger halo (with reversed colours) around the antisolar point.
No light is refracted into the region between the primary and secondary rainbow, and this dark region (shown below in a photo by L.T. Hunter) is called Alexander’s band, after Alexander of Aphrodisias, who first discussed it in around 200 AD, in his commentary on Aristotle’s Meteorology.