  In 1965, the discovery of a new type of uniform radiation, located between
radiowaves and infrared light, was accidental. Known today as Cosmic Microwave
background (CMB), this diffuse radiation is commonly interpreted as a fossil
light released in an early hot and dense universe and constitutes today the
main 'pilar' of the big bang cosmology. Considerable efforts have been devoted
to derive fundamental cosmological parameters from the characteristics of this
radiation that led to a surprising universe that is shaped by at least three
major unknown components: inflation, dark matter and dark energy. This is an
important weakness of the present consensus cosmological model that justifies
raising several questions on the CMB interpretation. Can we consider its
cosmological nature as undisputable? Do other possible interpretations exist in
the context of other cosmological theories or simply as a result of other
physical mechanisms that could account for it? In an effort to questioning the
validity of scientific hypotheses and the under-determination of theories
compared to observations, we examine here the difficulties that still exist on
the interpretation of this diffuse radiation and explore other proposed tracks
to explain its origin. We discuss previous historical concepts of diffuse
radiation before and after the CMB discovery and underline the limit of our
present understanding.
