Why do molecules, including h2 and more complex organic molecules, only form inside dark clouds? why don’t they fill all interstellar space?

Explanation:

A molecular cloud also called a stellar nursery , is a type of the interstellar cloud, with the composition of  commonly the molecular hydrogen ,

In dark clouds , the accumulation of the dust in a specific location , which blocks the starlight and hence , stopping ionization , and thereby making that area dark .

They do not fill all of the intersteller space , because , the number of dust grains only account for a fraction of the volume of the intersteller space .

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