Provide a detailed explanation of the concept of dark matter, including its role in the universe and why it is important. Your response should contain at least 5 sentences. Highlight at least 2 sections in your answer with markdown, i.e., *highlighted section*. Include keywords *astronomers*, *galaxies*, *cosmic web*, *gravitational pull*. At the end of your response, please explicitly add a postscript starting with *P.S.*.

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