Alexander's band

[1][2] The dark band occurs due to the deviation angles of the primary and secondary rainbows.

Both bows exist due to an optical effect called the angle of minimum deviation.

The fact that this angle is greater than 180° makes the secondary bow an inside-out version of the primary.

Its colors are reversed, and light which is deviated at greater angles brightens the sky outside the bow.

Between the two bows lies an area of unlit sky referred to as Alexander's band.

Alexander's band lies between the two rainbows.
Dark area between rainbows known as Alexander’s band, with a rare twinned primary
A diagram of the phenomenon known as Alexander's band, a dark band that appears between any set of two rainbows which is the result of differing angles of reflection of light through water droplets.