There are several things you might want to change.
1. Place the media inside a hollow container, e.g. a box or such. It can be
very difficult for POV-Ray to properly place the sampling of the intervals
when the rays aren't stopped at an object somewhere.
2. Also note that scattering media also absorbs light, so objects further
away will be less visible. With Point 1 this is the reason why the
background shows white, using a simple box (hollow and with pigment{rgbt 1}
to make the surface invisible) solves that.
3. Why do you set assumed_gamma to 5.0?! A normal screen has 2.2 or so, and
the Docs advise to use 1.0.

That should get you started.

Regards,
Tim

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

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