Ylem is a term which was used by George Gamow and his associates for a hypothetical original substance or condensed state of matter, which became subatomic particles and elements as we understand them today. It reportedly comes from an obsolete Middle English word that Gamow came across while thumbing through a dictionary, which means something along the lines of "primordial substance from which all matter is formed." Restated, the Ylem is what "thing" Gamow, et al, presumed to exist at least immediately before the instant of the Big Bang.