Galaxies have groups {galactic cluster}|. Cluster has diameter 10 million light-years, has thousand galaxies, and has even dark-matter and hot-gas distribution. Hot gas radiates x-rays. In early universe, radiation energy was high compared to gravity, so mass fluctuations were small. Only later did mass differences increase with time. Over distances of 1,000,000,000 light-years, galaxy density varies by 1/10. At larger volumes, grouping decreases, so universe is homogeneous. Early galaxies clustered more than dark matter.
For x-ray emitting galactic clusters, temperature varies directly with mass.
Physical Sciences>Astronomy>Galaxy>Kinds
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Date Modified: 2022.0224