Rainbows are not simple! It says that white sunlight is reflected back from inside/under the rainbow, but not above/outside the rainbow (until we get to the colour-inverted secondary rainbow).

[Edit: deleted repeated info about Alexander's Band]

It actually is brighter. The dark region beyond the rainbow is called Alexander's Band. Good explainer of the physics

White light is reflected back to the observer below the rainbow following a single lossy reflection from the back of each raindrop, so it is brighter here.

Above the secondary rainbow, white light is reflected back to the observer following a more lossy double reflection, so it is not as bright.

Between the primary and secondary rainbow, no light can be reflected back to the observer by the back of the raindrop, forming Alexander's Band.

A great explainer

This seems oddly familiar. Is it in Turkey?

Muff Diver