Use reasoning to answer the following question.
The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived from theory. The theory of dark matter was postulated to account for the variance.\n\nAnswer this question: what is the rotation curve of a spiral galaxy?

a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy 's centre
To answer the above question, the relevant sentence is: The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre.

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Final answer: a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy 's centre