Star-forming clouds appear dark in visible-light photos because the light of stars behind them is absorbed by interstellar dust.
Interstellar dust is composed of various elemental compounds such as oxygen, carbon, iron, silicon, and magnesium. It is caused by the death of stars, which produce metals during their lifetimes before exploding or blowing off their outer layers. Typical interstellar dust grains are much smaller than the dust we are familiar with on Earth, resembling more closely candle flame soot. Because they are roughly the same size as the wavelength of blue light, they absorb and scatter UV and blue light far more efficiently than red light.
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