This topic covers 2 threads, each of which may contribute to the apparent need for ‘Dark Matter’:
1. Misuse of Newton's Theory.
When Physicists, are calculating
The force between two bodies, with masses, 'M1' and 'M2', is given by
F = M1 x M2 x G
However, many physicists don't realise that this formula is only correct if both of the two masses are spherical and do NOT intersect oneanother.
This section also decribes what to do of two sperical bodies overlap.
When one or both are NOT both spherical, physicists should refer to the last Scholium in Section 12 of "Newton's Principia, Book 1." Essentially he says that:
if the bodies are NOT spherical then you cannot use the formula, without some further adjustments, which the reader must derrive.
The result of NOT following his advice is that, in calculations involving disc shaped galaxies, clusters
sheets etc, values like 'the total forces experienced when they overlap' are grossly under estimated.
This may help explain the apparent need for some 'magical' force, often thought to be due to 'Dark Matter'.
2. Ignored Hydrogen Atoms.
Inside the galaxy, most of the hydrogen has been attracted into individual stars as fuel for their fusion into helium. It is this fusion that provides all the light that we see being reflected around and flowing out from the galaxies.
Most hydrogen atoms lie beyond the visible outer boundary of the galaxy, where there are no visible stars. Hydrogen atoms and molecules do not reflect light and so these are generally ignored, despite the fact that they have gravitational atraction.
A lot of the hydrogen just outside the outer boundary of the galaxy, must be sufficiently dense to be close to triggering nuclear fusion, which would create a new star with a significant mass. (The numer of such new stars in a galaxy is roughly of the order of one per year per galaxy.) Like most of the rest of the mass in a galaxy, this star then migrates slowly towards the centre were it either dies or is swallowed by a Black Hole..
Some of the hydrogen results from stars that have started fusion, but fail to retain a minimum neccessary rate of fusion; this leaves some pockets of hydrogen density inside the galaxy. (Ref????????)
All of this hydrogen' naturally becomes part of the 'Dark Matter'.
Both of these situations cause effects that have been attributed to Dark Matter.