The term 'Matter in the Big Bang Theory' refers to the matter that was created during the Big Bang, which is the event that is believed to have marked the beginning of the universe. YLEM is a four-letter acronym for the primordial matter that was created during the Big Bang, which stands for 'Youngest Light Element Abundance Matter'. This matter is composed of the lightest elements, such as hydrogen and helium, and is believed to be the building blocks of the universe.