There are two major competitors for the title. Materialism says that the universe is made entirely of matter. Matter, for philosophers, includes energy and anything in the physical sense. Others thought it was fire. Others still thought the universe was composed of invisible particles which were neither created nor destroyed called atoms.
Idealism is the other competitor for the title.
Idealism says that the universe is made of the spiritual or the mental, which stuff they refer to as idea or the ideal. Early Greeks also had a variety of ideas regarding what particular brand of ideal constituted the universe. Others saw it as a sort of life force. Others still saw it as the perfection behind the flawed world we perceive. Modern idealist philosophers talk in terms of a world of persons, or a world of qualities.