  In the presence of magnetic helicity, inverse transfer from small to large
scales is well known in magnetohydrodynamic (MHD) turbulence and has
applications in astrophysics, cosmology, and fusion plasmas. Using high
resolution direct numerical simulations of magnetically dominated
self-similarly decaying MHD turbulence, we report a similar inverse transfer
even in the absence of magnetic helicity. We compute for the first time
spectral energy transfer rates to show that this inverse transfer is about half
as strong as with helicity, but in both cases the magnetic gain at large scales
results from velocity at similar scales interacting with smaller-scale magnetic
fields. This suggests that both inverse transfers are a consequence of a
universal mechanisms for magnetically dominated turbulence. Possible
explanations include inverse cascading of the mean squared vector potential
associated with local near two-dimensionality and the shallower $k^2$
subinertial range spectrum of kinetic energy forcing the magnetic field with a
$k^4$ subinertial range to attain larger-scale coherence. The inertial range
shows a clear $k^{-2}$ spectrum and is the first example of fully isotropic
magnetically dominated MHD turbulence exhibiting weak turbulence scaling.
