  The conjecture that dark matter in galaxies is mostly cold fractal gas is
developed in the more general context of the thermodynamics of the ideal
isothermal gas subject to gravitational instability. This simple gas model
already contains the contrary ingredients able to prevent an asymptotic
equilibrium: any growing gravothermal singularity evaporates in a finite time,
and any tendency to uniform gas is gravitationally unstable. The paradox is
simply resolved by allowing fractal states, which are then scale-free and
steady {\it in average}, but non-differentiable and time-dependent. If we apply
to clusters the lessons learned with galaxies, we are led to the conclusion
that gas in clusters at a temperature much below the virial temperature should
also adopt a fractal structure and become inhomogeneous. The same instrumental
biases acting at galactic scale and preventing the detection of the smallest
and coldest sub-resolution clumps in the fractal are then even more relevant
for cluster gas measurements. The large baryonic mass observed in the cluster
hot gas and the morphology-density relation suggest also gaseous dark matter in
spirals. If this dark gas component remains undetected in spirals, the same
instrumental biases should hold in clusters, where cooling hot gas disappears
from detection near the centre.
