(cosmology) the original matter that (according to the big bang theory) existed before the formation of the chemical elements WordNet 3.0 & Open English WordNet

Ylem is a hypothetical original substance or condensed state of matter, which became subatomic particles and elements as we understand them today. Wikipedia

In the Big Bang theory, the hot and dense plasma which made up the cosmos at the time of recombination in an early stage of its expansion and cooling, when the first atoms formed and photons decoupled. The ylem is regarded as the source of the cosmic microwave background. Wiktionary