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\newpage The difference between the two curves is known as the galaxy rotation problem. It means that either Newton's laws of gravitation are incorrect, or that there is some unobservable force acting within the galaxy leading to the observed effects. There are several proposed solutions to this, with the most widely accepted one being the presence of dark matter that is not mainly distributed in the centre of the galaxy, instead forming a dark matter halo. \subsection{Measurement of Galaxy Rotation Curves}  Original observation on the rotation rates of galaxies was pioneered by Vera Rubin in the 1960’s and 70’s. She inferred the existence of dark matter by calculating that galaxies would rip themselves apart at current speeds unless there was an unseen force holding them together. A spectrograph was used to measure the rotation of the galaxies, by looking at the redshift of the stars. Whilst doing this, the redshift due to expansion must be taken into account.