Keplerian motion- says that the line should eventually drop down but it doesn"t. So this means that there is more stuff then we can see. Seems to imply, that there is a big fuzzy group of stuff that interacts gracvitationaly. Its because of the rotation curve that we decided there is dark matter. No- there is mass in galaxy that we cant see. Obervation tell is that galaxies both spirals (from 21cm radio obs) as well as. At least 10 times as mich dark matter as luminous matter. Study of the hot gas in galaxy clusters tells us that there is mich more mass there than we can account for via the galaxies. Note: temp of the intracluster gas is related tot the total amount of the mass in the cluster. Observations tell us that galaxy clusters, using all of there methods, contain. At least 40 times as much dark matter as luminous matter.