Ylem is a term which was used by George Gamow, Ralph Alpher and their associates in the late 1940's 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 philosophical 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", and derives from the Greek hylem, "matter". Restated, the Ylem is the "thing" Gamow, et al, presumed to exist immediately after the Big Bang. Along with the ylem, there were assumed to be a large number of high-energy photons present, which we would still observe today as the cosmic microwave background radiation.