Theory or Process?
The word "theory" is a stumbling block in discussions of philosophy of science. The colloquial and scientific meanings have nothing in common. But there's a more subtle problem. "Theory" doesn't seem to mean quite the same thing in various sciences. Is "the theory of evolution" a theory in the same sense as the "theory of gravitation"? They are not. Evolution might better be characterized as a process, not a theory. That label would not in any way diminish the status or importance of evolution as a well-established fact of nature.
What's a theory?
Philosophers of science, and physicists, define "theory" as a well-established and well-tested unifying principle. In physics, theories such as gravitational theory and electromagnetic theory take the form of a compact set of equations from which one can logically deduce nearly everything in a particular subfield of physics. An established theory is the pinnacle and goal of the scientific enterprise, held in high esteem by scientists.
Biologists commonly refer to the theory of evolution. Here I may irritate some biologists by arguing that the theory of evolution is an entirely different sort of thing than theories in physics. Let's list some differences.
These differences don't in any way diminish the importance of evolution. Perhaps we should have a different name to characterize evolution. After all, what's in a name? By any other name the achievements of scientists who study evolution are in no way diminished, and the persuasiveness of their findings remains just as strong.
I suggest that we call it the "process of evolution", to distinguish it from the "fact of evolution". The gradual progression of living things through a succession of modifications is a fact of natural history well confirmed by an abundance of different and independent kinds of evidence. That's the "fact of evolution". The natural processes that brought this about are simply components of the "process of evolution".
There is no "force" of evolution or "principle" of evolution, and no single universal law of evolution. Evolution is the natural result of many other laws of physics, geometry, chemistry, etc. That's why one can't characterize evolution with a compact set of laws that you could write on the back of an envelope.
For those who like analogies, consider other processes of nature: mountain building, erosion, formation of meandering streams and rivers, sand dunes, ocean currents, atmospheric phenomena. They are all complicated when examined in detail. No single set of equations can predict the exact shape a mountain will have when it is fully formed. There are too many variables that can't be easily measured. Powerful computer programs are required even to predict tomorrow's weather reasonably well, and don't expect an accurate prediction for more than a week from now.
All of these processes have the feature of formation of orderly structures from lesser order, and eventually the breakdown of that order again. A mountain rises from flat land by physical processes, then erosion wears it down over a long period of time. A river may develop a wonderful pattern of sinuous looping meanders, then cut a new channel, abandoning an old loop to form a small lake. Sand dunes form, erode and reform. In the atmosphere, cyclones and anti-cyclones form and dissipate. The planets and the earth formed from a disk of gas and dust, and they now orbit the sun in orderly fashion and their moons orbit them with precise regularity. In all cases, when order increases, that process is driven by energy input. All of this is in strict accord with the laws of thermodynamics. 
A tornado is a dramatic example that illustrates natural processes at work to produce increased order. Calm air, gently rustling leaves, with no large temperature or pressure gradients, evolves into a whirling, violent and highly structured object, with characteristic behavior. Eventually it dissipates. Is this purposeful design? Is it in any sense an "accident" of nature? It it the result of "blind chance"? Posing these questions about such processes illustrates the inadequacy of these words to deal with them. Many of the philosophical issues that arise in the ID debate are embodied in this destructive maelstrom of whirling air. Perhaps that is a good metaphor for the entire ID controversy.
Perhaps avoidance of the word "theory" in these discussions could eliminate a lot of wasted words about irrelevant issues. Creationists will still object to anything that does not allow them to believe in the possibility of supernatural intervention in the natural order, but then it might be harder for them to conceal their real concerns.
Examples from Physics.
My suggestion to call evolution something other than a theory is not as radical as you might think. From my own field of physics, I note that we usually refer to "the laws of thermodynamics", but usually not the "theory of thermodynamics". Underlying thermodynamics is a microscopic theory of matter that we call "statistical mechanics". Though these have the character of theories in every respect, we don't generally call them that in everyday conversation. (However, textbooks sometimes do.) Failure to call them theories in no way diminishes their validity or importance.
Kepler's three laws of planetary motion might have been called a theory, just as we called Newton's three laws of mechanics a theory. But we don't ever speak of "Kepler's theory".
Of course we have atomic theory and nuclear theory and elementary particle theory. These are not sets of compact equations. Atomic theory came about as a result of the atomic hypothesis, at first controversial and somewhat nebulous in character, but as evidence accumulated it became refined and improved and finally achieved universal acceptance. If there's anything in physics that compares in character to the theory of evolution, this may come close.
The bottom line is that the word "theory" is not used consistently even in the physical sciences, so why should it be considered appropriate and necessary in biology?
Beyond the Facts.
One characteristic of most new physics theories is that they are models that include new conceptual framework, and often define fundamentally important concepts. These concepts "go beyond the facts" and are not concepts that are directly observable. This may shock some readers, but consider some examples.
Newton's theory of mechanics introduced a new definition of force to replace the various different concepts of force that went back to Aristotle. The definition wasn't even explicity stated, but was implicit in Newton's law F = ma. Then he went one step farther, by postulating that bodies with mass exert gravitational force on each other, even if there is nothing in the space between them. To many this seemed a radical notion, and many called this force "occult". Previously force was thought of as only something that was directly accessible to our senses. We "feel" forces. But Newton's use of the idea was more abstract. It was a conceptual tool for describing motion. We observe motion, but we infer that that motion was the result of a force, even though we don't direclty perceive the force. Force isn't something "material". It isn't an observed entity, but something invented to help account for things we do observe.
Later physicists formulated the concepts of momentum and energy. Again, these aren't material things that we can perceive with our senses. They are "accounting devices" to describe how things interact. Students sometimes think of these as "real", even to the point of thinking of energy as a "stuff" or "substance" that bodies can exchange, but physicists know better, and have gone far beyond that naive interpretation.
The early models of the atom treated it as a "miniature solar system", with electrons orbiting the nucleus like planets orbiting the sun. There was no direct evidence of this orbital motion, and visualizing the atom that way soon led to serious difficulties. The model "went beyond" the facts in imagining details that had not (and we later found, could not) be experimentally verified. Yet for many purposes the model worked. It was successful. It was, and still is, called "the Bohr theory of the atom". Remember that when physicists label something as a theory, that is the highest praise for the idea, indicating that it is successful, gives correct predictions, and is fruitful for furthering research.
The fruitful concepts of physics are several steps removed from observable entities, but they must have a clear logical connection with observables. As we said (at length) in the chapter on "reality", we do not fuss about whether such concepts are "real" in the sense that rocks and trees are, only that they fit into an explanatory logical structure rooted in observable facts. The concepts are our own inventions, and have a clear purpose. However, any concept that cannot be in any way connected to observables is inadmissable in science. That's what happened with the concept of the luminiferous ether. It was found that the ether was undetectable in any way, and was, in fact, logically unnecessary for describing anything we observe.
Evolution deniers charge that biological evolution isn't valid science because it has as-yet-unexplained "gaps" and because it "goes beyond the facts" or is "only an inference" from the facts. But that is characteristic of all theories, even the best and most successful theories. Creationists say that only facts should be taught in science courses. A collection of facts does not make up a science. It's the interpretation of facts that's important. Evolution opponents have sometimes proposed "disclaimer" stickers for biology textbooks. In the same spirit we would have to put a sticker in physics textbooks something like this:
So why doesn't the ID hypothesis qualify as a concept every bit as scientifically valid as energy or momentum? I suggest several reasons.
Does Science Explain?
A common complaint is that "Evolution doesn't explain anything." Well, of course it doesn't. I happen to agree with the old cautionary truism: "Science does not explain; it only describes." All of our laws and well established theories describe how things work, but do not explain why they work that way. I've said that in various ways throughout this document. Science does not answer "why" questions. Does Newton's physics explain gravity? No. Newton's laws describe how gravity behaves. Those who ask for "explanation" are yearning for something beyond descriptions, but science does not satisfy that desire.
There's little more to say on this point. Yet this is the sticking point for many anti-science folks. They want more than science provides. They want answers to unanswerable questions. This feeling goes way back to the early history of science, when science, mysticism and magical thinking were jumbled together. One such pre-scientific idea was "cosmic synchronicity", often paraphrased "as it is above; so it is below", meaning that the phenomena of the heavens are mysteriously linked to what goes on here on earth, and even within our bodies. Thus each organ of the body was "associated" with a celestial body which "ruled it", and celestial bodies also ruled human affairs. These ideas were central to astrology. This is the kind of "explanation" that some people even today still seeksomething that makes them feel "a part" of the larger cosmos. It's "feel good" philosophy. One manifestation of this is the obsession with mathematical patterns, such as the Fibonacci sequence and examples of Fibonacci spirals in nature and in art. A recent NASA photo of a galaxy showed a twisted spiral structure. Immediately some of the mysticaly-inclined folks saw a resemblance to the DNA molecule, and speculated that there was some sort of connection between the two. There is. It is geometry. A photo of the water stream behind a power boat propeller would show a similar spiral, but is it, too, mystically connected to the twisted galaxy and the DNA molecule? This is nothing more than the pre-scientific tendency to attribute significance to superficial analogies.
Some years ago I heard a parable, said to be from some Eastern religion, that seems appropriate here.
Here's another parable, which also seems relevant to our discussion.
These two parables make points that whole chapters of serious scholarly analysis cannot.
Endnotes. Here again, in citing examples of order emerging from disorder in nature, I have deliberately avoided the technical term "emergence" which biologists apply to these processes. For a very thorough discussion of emergence, see the entry in The Wikipedia. Emergence is the name applied to a process in which simple component parts combine to form a stable entity of greater complexity that has more order than the starting materials, and has new properties and behavior not found in its component parts. Examples abound in nature, as noted above. Emergence also describes several important steps in the early stages of life on earth. Like "evolution", this word can easily be misinterpreted. Emergence is not a law of nature. It is not a "force" of nature. In fact, to label a process "emergence" does nothing to help us understand it. That is why I saw no need to get technical about it. [Similarly, to say a body falls to earth "because of gravity" does nothing to help us understand gravity.] But the literature on the subject is well worth careful study, and it is an important search word when you want to find out more about the evolution of life on earth.
Intelligent Design Creationism: Fraudulent Science.