Galaxy rotation anomaly
Text from wikipedia:
If Newtonian mechanics is assumed to be correct, it would follow that most of the mass of the galaxy had to be in the galactic bulge near the center and that the stars and gas in the disk portion should orbit the center at decreasing velocities with radial distance from the galactic center (the dashed line in Fig. 1).
Observations of the rotation curve of spirals, however, do not bear this out. Rather, the curves do not decrease in the expected inverse square root relationship but are “flat”, i.e. outside of the central bulge the speed is nearly a constant (the solid line in Fig. 1). It is also observed that galaxies with a uniform distribution of luminous matter have a rotation curve that rises from the center to the edge, and most low-surface-brightness galaxies (LSB galaxies) have the same anomalous rotation curve.
The rotation curves might be explained by hypothesizing the existence of a substantial amount of matter permeating the galaxy that is not emitting light in the mass-to-light ratio of the central bulge. The material responsible for the extra mass was dubbed “dark matter”, the existence of which was first posited in the 1930s by Jan Oort in his measurements of the Oort constants and Fritz Zwicky in his studies of the masses of galaxy clusters. The existence of non-baryonic cold dark matter (CDM) is today a major feature of the Lambda-CDM model that describes the cosmology of the universe.
Rotation curve of spiral galaxy M 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (white line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.
A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected under the presence of dark matter.
Comparison of rotating disc galaxies in the distant Universe and the present day
There is another possibility to explain the difference in velocity observed in these galaxies. Many experimental results on the effect of light on the weight of objects suggest that all matter emit gravity in all directions. Some of that gravity is absorbed by atoms and changed the velocity of these atoms. That seems to be a pushing force caused by gravity.
The galaxy shown on the right is surrounded by millions of other galaxies as seen in the picture on the left. Each atom of these galaxies also emit gravity. That gravity reaching the galaxy in the middle of the picture will exert a pushing force on all its atoms. But some of that gravity is blocked partially going through the galaxy. That causes an imbalance mostly on stars on the outside of the galaxy. They receive more gravity from the exterior that from the interior. The total forces looks like there was a kind of attraction coming from the center. That was the origin of the concept of black matter in the center of the galaxy.
At first observation, it seems that it would receive more gravity coming from the interior because it contains more atoms, but that is not so. The reason is because that galaxy is like a flat pancake, the gravity going toward one star at the edge is more intense in the same plane as of the galaxy itself. But the stars on the perimeter receives gravity not only in the same plane as the galaxy but also from all directions of space. The total amount when all the vector forces are added causes a bigger pushing force in the plane of the galaxy oriented toward the center of the galaxy. The next diagram explains that force.
Gravity emitted by the galaxy is going in all directions and there is more in the same plane as the galaxy.
The star in the red circle seems to receive more gravity in the same plane as the galaxy. But it also receives gravity from all directions as shown in the drawing at the left. The total vector forces is now directed toward the center of the galaxy as shown in the drawing at the right. That is the reason the velocity of that star does follow the inverse square root formula. We do not need any special black matter to explain this behaviour.