  The interstellar medium is structured as a hierachy of gas clouds, that looks
self-similar over 6 orders of magnitude in scales and 9 in masses. This is one
of the more extended fractal in the Universe. At even larger scales, the
ensemble of galaxies looks also self-similar over a certain ranges of scales,
but more limited, may be over 3-4 orders of magnitude in scales. These two
fractals appear to be characterized by similar Hausdorff dimensions, between
1.6 and 2. The various interpretations of these structures are discussed, in
particular formation theories based on turbulence and self-gravity. In the
latter, the fractal ensembles are considered in a critical state, as in second
order phase transitions, when large density fluctuations are observed, that
also obey scaling laws, and look self-similar over an extended range.
