Re: Is it possible to make yogurt without a starter culture?

Probably not.  In order to make yogurt one needs milk and two different 
bacteria.  Streptococcus salivarius subsp. thermophilus  and 
Lactobacillus delbrueckii subsp. bulgaricus.  Both of these organisms are 
present in yogurt you purchase at the local market but most likely not at 
optimum concentrations or proportions.

It is possible that raw milk or even pasteurized milk may contain some 
lactic acid bacteria but raw milk may also contain dangerous bacteria and 
pasteurized milk most likely contains spoilage bacteria.

If you want to make your own yogurt your best bet is to heat your milk 
and add a starter.  If you do not have a starter try adding some yogurt 
to the milk.