Spiral galaxies rotate, and the velocity of rotation versus distance from the center is known as the rotation curve. Our own Sun takes about 240 million years to rotate about the galaxy’s center.
M64, image credit: NASA, ESA, and the Hubble Heritage Team (AURA/STScI)
Almost always the rotation curves flatten out far from the center. This was not expected. If the mass is concentrated as the light in a galaxy, the velocity should continue to decline as one goes further out, according to Newtonian dynamics.
The flattening can be interpreted as due to the presence of dark matter, or alternatively, due to deviation from Newtonian dynamics and general relativity when accelerations are very low.
Now researchers have looked at a sample of over 100 distant, high-redshift galaxies and they find the rotation curves look much more like one would expect with only Newtonian gravity. So is dark matter not important at higher redshifts? Read more at:
Distant Galaxy Rotation Curves Appear Newtonian
72 Beautiful Galaxies 