None (Physics Material) Properties

It is 5 years too late, but I needed to know myself.

So for anyone still wondering, the default physics material is the same as when you create a new physics material. This defaults to:

3D Physics Material:

 Dynamic Friction: 0.6
 Static Friction: 0.6
 Bounciness: 0
 Friction Combine: Average
 Bounce Combine: Average

2D Physics Material:

 Friction: 0.4
 Bounciness: 0

Note that you can change the default materials by going under Edit > Project Settings > Physics or Physics 2D > Default Material, if you for example want to add a little bounciness to none-physics material objects.