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• n  (cosmology) the original matter that (according to the big bang theory) existed before the formation of the chemical elements

• In the beginning, says one school of cosmology, there was "ylem": a featureless mass of protons and neutrons containing all the matter in the universe.
• The original nuclear reactions were complete in a few minutes, and they generated so much energy that the ylem (from Greek, original matter) blew up with cosmic vigor.