Eli5: Why do you see a very dark gray, but not quite black color in pitch-black darkness?

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Try it. If you look into pure blackness you don’t really see “black,” but this kind of fuzzy dark gray

I would have expected I would see pure vantablack or the blackest black ever in my life, but why does it happen that the total absence of light is in fact, a very-dark-but-not-quite-black-colour.

In: Biology

Because your vision is partially relative to other nearby objects.

As an example, a “black” asphalt road under direct sunlight is sending considerably more light into your eye than a “white” object under dim illumination would be. Actually, a vantablack-colored object under bright direct sunlight (reflecting 1/2,000 of at most 100,000 lux illumination = at most 50 lux) is approximately as bright as a white object under indoor lighting at night (reflecting most of ~50 lux illumination). But the asphalt still looks black because it’s much dimmer than surrounding concrete, trees, metal, etc, while the dimly-lit white object looks brighter than surrounding objects. (This is why the Moon appears bright white in the night sky, despite the lunar regolith [actually being a rather dark grey](https://en.wikipedia.org/wiki/Lunar_soil#/media/File:Lunar_Regolith_70050_from_Apollo_17_in_National_Museum_of_Natural_History.jpg). The moon actually reflects about as much light [about 10%] as a forest or an area of wet soil would, but appears bright relative to the blackness of space.)

In pitch darkness, your vision is dominated by random accidental activations of the cells in your eyes (mostly just due to the tiny heat-based vibrations of the molecules that detect light). Without contrast, those activations aren’t darker than the surrounding field, resulting the dark grey (called [“eigengrau”](https://en.wikipedia.org/wiki/Eigengrau)) that you describe.