why the outer stars in the galaxy move with the same speeds of inner stars in the same galaxy

According to Kepler's equations of planetary dynamics, the inner stars in a galaxy should be rotating fast than the outer stars. However, the stars in a galaxy seem to disobey this rule.

 

This is because, according to modern accepted theory, there is a large amount of mass in the galaxy's empty space that is unaccounted for in the Kepler's laws.

This mass is given the name "Dark Matter". If we incorporate the mass of drk matter, then we will come to know that the stars should move with same speeds.

 

NOTE: There is another theory which explains the constant speed of the stars. However, this theory is beyond the scope of school and higher school students. This theory is called "Modified Newtonian Dynamics (MoND)". According to this theory, the velocity of stars is independent of the distance of them from the centre of the galaxy.