Intelligent Design proponents who claim that they have a ‘theory’ often formulate it in the form of “evolutionary/Darwinian mechanisms cannot explain X”. When pressed for a scientific theory, it quickly becomes obvious that ID is scientifically vacuous.
Easily the biggest challenge facing the ID community is to develop a full-fledged theory of biological design. We don’t have such a theory right now, and that’s a problem. Without a theory, it’s very hard to know where to direct your research focus. Right now, we’ve got a bag of powerful intuitions, and a handful of notions such as ‘irreducible complexity’ and ‘specified complexity’-but, as yet, no general theory of biological design.
Paul Nelson, Touchstone Magazine 7/8 (2004): pp 64 – 65.
For Paul’s explanation see this link