Of course there exists an intelligent designer...he is called 'Homo sapien sapien' and he is rather easily physically apprehended. The problem we have is the lack of evidence for and information about any other intelligent designer. That is not to say that this designer does not exist, just that we have no information about his design habits, intent or practices (without making some huge assumptions).
If this putative designer happens to design exactly as we do then we should be able to, at least in some cases, discern his/her/its work. That is as long as that designer doesn't use manufacture methods we have no experience with (much of how we identify design is through obvious manufacture) and doesn't do a perfect job of emulating nature. If this alien designer thinks differently than we do and consequently has designs that do not resemble human design we will have a heck of a time discerning his/her/its work. In addition to that, what if nature *can* create phenomena that looks every bit as organized and complex as an intelligent designer (and there is no evidence that it can not)?
It seems to be a problem that extends only as far as a few narrow-minded ideologues. A single strand of DNA provides ample evidence of intelligent design. Science is beginning to quantify the amount of information processed through genetic material. How do you suppose it compares to the amount of information needed to build an automobile?
The presence of information is one evidence of intelligence. Do you think the presence of information is too hard for science to define and detect?
Funny how indirect evidence suits the purposes of science in every case except when it points to intelligent design. Why the blind spot?