Intelligent Designer of the gaps

The ID case

1. Complex biological system is observed.
2. Explanation for said biological system is lacking.
3. It is concluded that an Intelligent Designer did it, but we don't know how the Intelligent Designer did it
   

In step 3 the proponent of intelligent design has made the Intelligent Designer a tautology for "I don't know" and attempted to pass that off as an explanation. This is the same argument that has been made throughout the ages; identify some mystery and then invoke God as the explanation ... that is until an actual explanation comes along.

Why is it thundering? Because Zeus is making it thunder.
How did this complex biological system develop? An intelligent designer developed it.