BA #1 The Road

One thing we don't know about the situation that they are in is the extent (range) of the catastrophe.  They don't know any details about the condition of the "south", the place to which they are headed for warmer weather and a potentially, but not guaranteed, better life.

 The secrete of the extent of the damage is directly correlated to the secrete of what caused the destruction and decay in the first place.  If we knew what caused the damage we would be able to calculate the extent of it.  They are taking a dangerous and possibly fatal journey to a place that may be just as severely in ruins as the place at which they are now.  We are led only to see the extend of the damage by how far they have traveled south and we know little to nothing of where they are.

Having the secrets is what makes the story appealing, you do not need to know exactly what happened to understand why things are happening.  If Cormac McCarthy described everything intricately then we would have nothing to discuss.  If we knew every detail then it would be a journal of the two characters lives.