All double rainbows are separated by such a band. The bright primary bow (on the left) is formed by light reflected once inside raindrops. The less-bright secondary bow (on the right) is formed by light reflected twice inside raindrops. These reflections, single vs. double, scatter light in opposite directions. In between the two rainbows, light is simply not bent toward the observer, so it looks dark.
And that's how rainbows can suck light out of the sky. Find more examples in the Realtime Rainbow Photo Gallery! www.spaceweather.com