The Long Answer:
|The Short Answer: The answer is that intelligent design theory itself is the evidence for the intelligent designer.|
Well, it's pretty simple: intelligent design theory works by looking for the tell-tale signs of intelligent design in biology. We know and understand the sort of information produced by intelligent agents. We can then look for that sort of information in biology. These tell-tale signs include high levels of specified complexity, which we know is always the product of intelligent design. Thus, when we find evidence for specified complexity, we have evidence that there was intelligent designer! Intelligent design theory need not provide some sort of external evidence for an intelligent designer because the theory itself
a legitimate rationale for concluding that the cause of intelligent design was at work.
It should be noted that many theories in science have been accepted even if we didn't totally understand the mechanism, or have direct observational evidence of the mechanism. In fact this is very common in science. For example, plate tectonics was accepted because of overwhelming evidence of continental drift (
i.e. fit of continents, earthquakes and volcanic activity along plate boundaries, oceanic spreading centers, paleomagnetic evidence for plate movement, matching fossils on plates on opposite sides of ocean). However, scientists today still do not fully understand the mechanism that allows for the plates to move! (they have some ideas, but it still isn't fully understood.) Nonetheless, no one doubts that the plates move--there is too much evidence that they do move, even if we don't fully understand how. What about gravity. We can measure its strength, put it into equations, and make predictions about what it will do. But does anyone understand the physical reasons why matter is attracted to matter? No. Nonetheless, apples still fall from trees, and life bears the marks of intelligent design.
Finding the tell-tale signs of design itself is enough to infer design. Indeed, archaeologists often find evidence of designed objects, but they don't otherwise have any idea who designed them. But because they look designed, no one doubts they were. Similarly, if we have overwhelming evidence for intelligent design, we can accept design even if some think we don't fully understand the identity of the designer or don't otherwise have any independent evidence for the designer.