Here is how intelligent design theory works:
1. The ways that intelligent agents act can be observed in the natural world and described.
2. When intelligent agents act, they tend to produce high levels of "complex-specified information", and in our experience, complex- specified information is always the product of the action of intelligent design.
3. "Complex specified information" is basically a scenario, or circumstance which is unlikely to happen through natural processes (making it complex), and conforms to a specific pattern (making it specified). Both language and machines are good examples of things with high levels of complex-specified information.
4. When we look at biology, very complex machine-like entities exist, which must be exactly as they are, or they cease to function properly. They are specified, becaues they conform to a particular pattern of arrangment organization which is necessary for them to function, and complex because they have an unlikely arrangement of many interacting parts.
5. The high level of complex-specified information in these biological machines makes them "irreducibly complex"--they have many interacting parts (making them complex) which must be EXACTLY as they are in order for the machine to work properly (making them specified), and any change in the nature or arrangment of these parts would destroy their function and make the machine stop working, making them "irreducibly complex" (they could not be any less complex and still function).
6. These "irreducibly complex" structures cannot be built up through a Darwinian evolutionary process, because Darwinian evolution says that a biological structure must be functional along every small-step of its evolution, and "reverse engineering" of these structures shows that they cease to function if changed even slightly.
7. Because there is no known natural mechanism to explain the origin of these "irreducibly complex" biological structures, and because they exhibit complex-specified information, a quality known only to be produced by intelligent design, we conclude that these irreducibly complex structures are intelligently designed.
Thus, if we recognize the tell-tale positive predictions of intelligent design, e.g. specified complexity, then we are justified in inferring design. Where the designer came from or how the designer originated is not a component of the argument for design. An simple example might help explain.
Let's say you are walking in a field and you find a TV set. Now you don't necessarily know anything about who or what designed that TV set, but you can tell it is designed because, at some fundamental level, it exhibits specified complexity. You don't have to know where the designers came from--a) they could have evolved, b) they could have been designed, or c) they could have existed without a cause from eternity. It doesn't matter: you can justifiably infer intelligent design without having to explain or know how the designer arose.
The implication in this objection, however, is that somehow option (c) is not a viable option because many people cannot explain the origin of God, who many believe to be the Designer. This is a religous / theological objection to intelligent design because it deals with philosophical statements about the designer that have nothing to do with the emprical study of detecting design. This objection delves into philosophy and asks if an uncaused designer would be philosophically acceptable assertion. (note that intelligent design theory doesn't necessarily say anything about how the designer arose, but let's just say for the sake of responding to the question that philosophically, we are employing option (c) and that the designer did exist eternally in the past, and has no "origin.")
Since this is a theological question, we can give a theological answer (which has nothing to do with the scientific theory of intelligent design). Is it really true that, philosophically speaking, it isn't acceptable to invoke God as an explanation for the origin of the universe unless we can somehow account for the origin of God? For the Christian theist, there is no explanation for the origin of God, for God is by definition a Being existing outside of space and time eternally in the past, present, and future, from Whom all things which were created have come, who has no origin:
Psalm 93:2: "Your throne was established long ago; you are from all eternity."
Proverbs 8:23: "I was appointed from eternity, from the beginning, before the world began."
John 1:3: "Through him all things were made; without him nothing was made that has been made."
When it comes to worldviews, every worldview is left with some kind of an uncaused and assumed entity at the foundation of that world view. For theism, the uncaused cause is the origin of God. For atheism, the uncaused cause is the origin of the universe. In essence, every worldview necessarily has unknowns or uncaused causes at the very beginning. When asked where God came from, the theist may answer, "I don't know", but when asked where the universe came from, the non-theist must also then answer, "I don't know".
The question thus does not come down to, "Is it possible that God could have been uncaused and existed infinitely in the past?" (The answer to that question is, "yes--philosophically, it is possible that God is an uncaused cause, or an unmoved mover.") Beause all worldviews have assumed uncaused causes at their very beginning, this question comes down to, "Whose uncaused cause seems the most reasonable?"
Some non-theists may try to avoid this unknown through a theory of cyclical universes, where our universe came from a previous universe, or theoretically exists inside some other universe, but all of these explanations still regress back to the question, "what started off the chain of events?". The non-theist must answer, "I don't know", but the theist has an explanation for one more thing than the non-theist: the origin of the universe. We may not be able to understand the "origin" of "God", but we know that space-time and energy-matter can come from a superpowerful Being. Using God as an explanation for the origin of the universe is thus an acceptable philosophical inference which actually has a larger explanatory power than a model which doesn't invoke God and leaves the origin of the universe unexplained. Theism thus provides a more philosophically acceptable uncaused cause: God. Since the universe appears to have been designed by an intelligence, postulating a super-Intelligence (whose origin is unknown or, the case of Christian theism, who has no origin) who created the universe, seems more reasonable than to postulate a designed universe that looks that way for no apparent reason.