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 by applying gravity theory to the matter observed in a galaxy.
Source : Wikipedia
It was first noted in the 1930s that the difference in the predicted rotation curve (using extrapolated star count estimates and the strength of the) correlated with data from observations, do not match. With errors typically in the range of 20 - 50%
Subsequent measurements have reliably found the same discrepancies. It was these observations that led to the hypothesis that there must be some kind of (invisible) gravitationally active material within galaxies that influences their rotation speed - so called
Other theories are that Newtonian gravitational calculations do not (for unknown reasons) hold on a galactic scale, or, that gravity itself may not be constant.
Also see :and
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