Why a whole galaxy is turning with unique angular speed

Here comes my assumption:

Imagine, there is an additional gravity force which increases proportional to the distance: F = c m M r

c: constant

m: small mass (star)

M: big mass (galaxy)

r: distance between m and center of M

equal of centripetal and centrifugal force:

c m M r = m r w^2

w: angular speed

division by m and r : c M = w^2

This is what can be observed: the angular speed inside a galaxy is just a function of its mass.

Maybe this force is nothing else than the bending of the space by the time.

e.g. gravity also bends the space.

IMHO this formula is not more fantastic than worm holes, dark enery and matter, multiple universa and so on.

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Some insects - called the human race

lost in time and lost in space