Wikipedia has this to say about the rotational speed of the milky way:<p>&quot;Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name &quot;dark matter&quot;.[34] The rotational period is about 240 million years at the position of the Sun.&quot;