What is Intelligent design?
"The theory of intelligent design (ID) holds that certain features of the universe and of living
things are best explained by an intelligent cause rather than an undirected process such as natural selection. ID is thus a scientific
disagreement with the core claim of evolutionary theory that the apparent design of living systems is an illusion that can be adequately
explained by only natural causes.” 
ID should not be confused with biblical creationism as ID looks for intelligence in nature and biblical creationism starts with the Bible account of creation and interprets nature from that standpoint. ID does not specify the age of the earth or that the designer is the God of the Bible. 
What are the sources of opposition to ID?
There is not just one source of opposition. Atheists strongly oppose ID because intelligence means there must have been a designer, a concept that they hate because of its implications, and therefore they prefer to view everything as coming about by materialistic evolution. Some may oppose ID from the mistaken view that science must involve only naturalistic explanations. Others may resist change from the current evolutionary dogma that they have been trained to accept. Some may resist because of the negative propaganda against ID or their scientific associates oppose it. Still others may oppose it for pride, monetary reasons, and the fear of being expelled. The concept of a designer is not out of the realm of science if science is truly the objective search for knowledge wherever that search me lead.
Is ID Science?
ID is science because it is “knowledge about or study of the natural world based on facts learned through experiments and observation”  and utilizes the scientific method. Actually, there is not just one scientific method; however, the one commonly used utilizes a four step process. That process is indicated below as applied to ID:
1. Observation(s) - “Certain features of the universe and living things” have the appearance of being designed
2. Hypothesis - “Certain features of the universe and of living things are best explained by an intelligent cause rather than an undirected process such as natural selection”
a. Find what are the indicators of design by intelligent agents
(1) Irreducible complexity - “a single system composed of several well-matched interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.” 
(2) Specified complexity - meets all the following requirements
(a) not due to natural law
(b) is highly improbable
(c) conforms to an independently given pattern 
(3) Inference - choose among the alternative competing theories and select the one that provides the best explanation  
(4) Anthrophic principle - intelligent design indicated by the remarkable fine- tuning of “over a dozen universal constants (including the strength of gravity, the strength of the electromagnetic force, and the ratio of the masses of the proton and electron)” for life. “If any of these constants were even slightly different, the universe would be uninhabitable.”  
(5) Other forms of design detection
b. Compare empirical evidence for selected features of living things and/or the universe against one or more of the design indicators in item 3a.
4. Conclusion(s) - state the results and in addition preferably make predictions that may also be tested.
examples of intelligent design include the bacterial flagellum,  the biochemistry of animal sight,  information in the DNA transcending
natural laws,  biological machines in cells,  the extreme improbability of one functional protein being made by chance,  the
blood coagulation cascade,  and the fine-tuning of universal physical constants to support life.  
One criticism of ID is that its adherents have not performed any experiments. The scientific method does not require that scientists perform experiments if the empirical data can be obtained by research of scientific peer reviewed publications. In recent years, there have been a tremendous increase in the knowledge of biological systems readily available to ID researchers.
The New World Encyclopedia reference below provides a good history and summary of intelligent design. It includes ideas on intelligent design from some of the leading ID theorists.
 Intelligent Design,
 Intelligent Design, http://www.newworldencyclopedia.org/entry/Intelligent_design
 Science definition, Merriam-Webster Online, August 20, 2014, http://www.merriam-webster.com/true-or-false/index.htm
 Meyer, Steven, Signature in the Cell: DNA and the Evidence for Intelligent Design, (New York: HarperCollins Publishers, 2009), 154-159
 The Privileged Planet, DVD, (Illustra Media, 2004), http://www.Illustramedia.com
 Behe, Michael, Darwin’s Black Box, (New York: The Free Press,
 ibid., 18-22
 Meyer, 341
 Unlocking the Mystery of Life, DVD, (Illustra Media, 2002), Chapter 10, http://www.Illustramedia.com  Meyer, 212
 Behe, Chapter 4
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