In physical cosmology, a cosmon or cosmonium is a hypothetical form of matter. The idea was originally proposed by Georges Lemaître, who suggested the concept of a "primeval atom" 1946, leading up to the theory of the Big Bang. He illustrated the idea by imagining an object 30 times larger than the volume of the sun containing all the matter of the Universe. Its density would be around . In his view, this occurred somewhere between 20 and 60 billion years ago.