Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. These gas clouds leave numerous absorption lines in a distant quasar 's spectra , together called the Lyman-alpha forest . The above image depicts one possible computer realization of how Lyman-alpha clouds were distributed at a redshift of 3. Each side of the box measures 30 million light-years across. Much remains unknown about the Lyman-alpha forest , including the real geometry and extent of the clouds , and why there are so many fewer clouds today .
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