"This thing ready, cat?"
"Whenever you are."
"Right, let's get on with it then..."
Let's start of with: Why Dark Matter and What is it?
Dark Matter--Some heavy stuff we can't detect floating around in space. Cosmologists believe its existence is necessary because otherwise all their fancy schemes fall apart. Specifically, the calculable mass of matter in the universe is insufficient to account for the gravimetric effects observed on numerous fronts. Therefore, there must be matter that we are not detecting. I know that sounds like an oversimplification but that's really what it comes down to if you boil away the hype.
Remember when I mentioned circular assumptions? Dark Matter may turn out to be one of the greatest circular assumptions of all time. The entire Dark Matter discussion is predicated on the assumption that our current understanding of gravity is correct and that Einstein's theory of general relativity is also correct. Further, Dark Matter is automatically a metaphysical theory, rather than a scientific physical one, because as yet we can detect no evidence and may never be able to beyond inference. This is a good example of why metaphysics is a fine and useful tool and should not be shoved into the gutter as many scientists today try to do. Without the metaphysical postulation of Dark Matter, we cannot derive hypotheses or experiments to look for Dark Matter and could therefore never elevate it into the category of scientific investigation. In other words, without the roots of metaphysics—the schemes and dreams of 'what if'—we could not make scientific progress. Despite its pre-scientific trappings, Dark Matter is a metaphysical or pseudo-scientific theory (or set of theories) at this point. There is nothing wrong with this so long as we understand that it is in the "pre-science" stage rather than running to the media, pointing to the sky and yelling "Lo, I have found a new thing, proven by science!" (Small Charles Fort reference there for your amusement.)
Next Time: MACHOs!