  We argue that clusters of galaxies have an intergalactic medium, which is
permeated by strong magnetic fields and also has a contribution of pressure
from cosmic rays. These two components of total pressure are probably highly
time dependent, and range probably between 1/10 of the gas pressure up to
equipartition between gas pressure and the sum of the two other components.
Radio galaxies are likely to provide the main source for both magnetic fields
and cosmic rays. In this concept it becomes easy to understand the occasional
mismatch between the total mass inferred from the assumption of hydrostatic
equilibrium derived purely from gas, and the total mass derived from lensing
data. We also suggest that the structure and topology of the magnetic field may
be highly inhomogeneous - at least over a certain range of scales, and may
contain long twisted filaments of strong magnetic fields, as on the Sun. The
analogy with the interstellar medium may be fruitful to explore further, where
we do not know where magnetic fields come from, but suspect that the cosmic
rays derive from supernova explosions. In such an analogy it becomes useful to
refer to "radio galaxy explosions" in clusters of galaxies. A full scale
exploration of all the implications, especially of the notion that occasionally
complete equipartition may be reached, is a task for the future.
