every one will give example of tossing a coin.but the coin shows head or tail according to physical law of force given for tossing. if you have a good machine, we can give desired force and get head or tail that is what ever we want. it is not uncertain event, if we measure exact force...
but it is said that stars in the arms are moving at fast rate. the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?
if there is dark matter every where, why it is noticed near galaxies? why can't we do some very sensitive lab experiments of gravity to find the effect of dark matter?
why galaxies are having uneven distribution of stars, in the form of arms? actually stars should have been like a thin disk like distribution of stars.