Galaxies are big, very big, on average about 100,000 light years across. Galaxies form in the shape of a bared spiral. As galaxies rotate we would expect the spiral arms to wind up over time and its spiral structure to become obliterated. A spiral galaxy rotates much like a wheel where the spokes don’t wind up. The problem is the periphery is rotating so fast that the spiral arms would be expected to wind up and the galaxy fly apart from centripetal force. This is not what is observed. Spiral galaxies maintain their spiral shape over billions of years. The question is why?

Mainstreams answer to this question is that most of the mass of the galaxy is in an invisible disk outside the galaxy and it’s the gravity of this ‘dark matter’ that hold the spiral in shape.

There is a simpler explanation that does not require anything ‘new’ like ‘dark matter’ to be added. Unfortunately, the simpler explanation takes longer to describe.

The problem originates because scientists are thinking in classical Newtonian terms. If you think about the problem in terms of Relativity, which they should be as it’s not a Newtonian problem as Newtonian gravity is only relevant on the small scale, there is no anomaly.

John Archibald Wheeler’s succinct summary of Einstein's theory of general relativity, “space-time tells matter how to move; matter tells space-time how to curve”.

A galaxy is big, very big; it is a very long way across, contains a vast amount of mass and does not exist in one single time frame. It is the densest in the centre becoming progressively less dense toward the periphery. Time is the most dilated at the centre becoming progressively less dilated toward the periphery. Distance (space) is less dilated at the centre becoming progressively more dilated toward the periphery. At any radius from the centre of the galaxy, stars follow a circular course as the galaxy rotates. The greater the radius, from the centre, the shorter a second and the longer a metre measuring rod. Speed is distance divided by time. The greater the radius, the shorter a second and the longer a metre measuring rod. The speed of stars at greater radius is decreasing as they travel ‘less’ long metres per ‘shorter’ second. The length dilation and time contraction factor changes in just the right manner in order to keep the ‘speed’ of rotation at any radius such that the spiral arms do not wind up. “Just the right manner” is no coincidence; it’s the constant ‘speed’ of light that stops a spiral galaxy from winding up. There is no anomaly.