Inter-galactic Magnetic Fields mass-density-equiv ?

[Widdekind]

The energy density of magnetic fields is [math]U \approx \frac{B^2}{\mu_0} = \epsilon_0 \, c^2 \, B^2[/math]. For an inter-galactic magnetic field strength of ~10^-12 T, this produces a mass-density equivalent of 10^-35 km/m³. This does not appear to be a principal component of the critical density.

[Widdekind]

http://www.physorg.com/news204298215.html