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1. Baryonic Matter Density

   Linked via "absorption line spectroscopy"

   The primary reservoir for the missing baryons is hypothesized to be the Warm-Hot Intergalactic Medium (WHIM). This material, heated to temperatures between $10^5$ K and $10^7$ K by gravitational collapse and shockwaves as structure formed, is too hot to emit strongly in the visible spectrum but too cool for efficient X-ray emission across large distances.
   Detection relies on absorption line spectroscopy against bright background […