I generally agree with Davin on this subject, although I will concede that there are some areas of "normal" science where the line gets a bit fuzzy.

To me--and I will readily admit I'm no expert in the field--the whole idea of "dark matter" has a faint ring of something complicated that is theorized to fill an uncomfortable gap in knowledge, like the convoluted crystal spheres of early solar system thinking.

The difference between a pseudoscientist and a real one though, is that a real scientist will drop a fuzzy theory like a hot potato if something better and simpler that fits the facts comes along. A pseudoscientist will tend to cling to his pet idea in the face of all new data.