We do not know. The best speculation these days is that the 'medium' was the physical vacuum. As many of my answers in the section, and my Sky and Telescope essay in 1996 suggest, there is much we do not know about the vacuum 'state'. Extensions of Big Bang cosmology developed by some physicists seem to attribute many very unusual properties and phenomena to this state. In any true fundamental way, no one really understands enough about space-time and the quantum dynamics of the vacuum state to be able to predict what the implications are for cosmogenesis. How one goes about testing such predictions is equally unknown.