It's like a Border.

Its part of the same optical illusion that creates the rainbow you see. The sky is not actually lighter on the inside of the rainbow, it just looks like it is

edit: i added the word “actually” for added clarity.

This is a good way to sus out a fake picture of a rainbow.

But what's it taste like?

the biome changed

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

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

The video confirms what i was saying. I am confused

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]

That is some S tier photography, well done

Not reflected, but REFRACTED. The light bends and make the brighter effect.

Rainbows involve reflection and refraction.