Star-forming clouds appear dark in visible-light photos because the light of stars behind them is absorbed by

Interstellar Dust

Explanation:

In the solar system Interstellar dust is a type of cosmic dust and unlike Interstellar gas they have large dust particles enabling them to block visible light. Unlike smoke and fog, they contain large number of closely packed clumps of atoms and molecules

interstellar dust

Explanation:

Dust particles interact with light through scattering and absorption. Reducing the amount of starlight received

The colour of the sky during the day is blue, because the blue light  is scattered by atmosphere particles.  The increased path length causes the change of colour of sun light . This is due to dust grains in the atmosphere, a similar process happens with interstellar dust

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