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The left end of the graph represents the power spectrum of density fluctuations over relatively large patches of sky (10,000 to 1,000 megaparsecs or Mpc), representing an era before galaxy formation. All three models produce power spectra compatible with fluctuations in the cosmic background radiation, but at small scal es--corresponding to galaxy formation--they differ.
A universe dominated by pure cold dark matter predicts fluctuations of greater strength, and therefore more and bigger clusters of galaxies than we observe; a pure hot dark matter-dominated universe predicts fluctuations of less strength, and thinner and smaller galaxy clusters than we observe. A model incorporating both cold and hot dark matter produces a power spectrum more similar to that predicted by the actual distribution of galaxies.
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