  In the interstellar medium, as well as in the Universe, large density
fluctuations are observed, that obey power-law density distributions and
correlation functions.
  These structures are hierarchical, chaotic, turbulent, but are also
self-organizing. The apparent disorder is not random noise, but can be
described by a fractal, with a deterministic fractal dimension. We discuss the
theories advanced to describe these fractal structures, and in particular a new
theory of the self-gravity thermodynamics, that could explain their existence,
and predict their fractal dimension. The media obeying scaling laws can be
considered critical, as in second order phase transitions for instance.
