Ylem ( or ) is a hypothetical original substance or condensed state of matter, which became subatomic particles and elements as we understand them today. The term was used by George Gamow, his student Ralph Alpher, and their associates in the late 1940s, having resuscitated it from Middle English after Alpher found it in Webster's Second dictionary, where it was defined as "the first substance from which the elements were supposed to have been formed."

In modern understanding, the "ylem" as described by Gamow was the primordial plasma, formed in baryogenesis, which underwent Big Bang nucleosynthesis and was opaque to radiation. Recombination of the charged plasma into neutral atoms made the universe transparent at the age of 380,000 years, and the radiation released is still observable as cosmic microwave background radiation.