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  Dark Matter Questions

the post:

I have some Questions regarding to Dark matter...

1) Is it true that Dark matter keeps galaxies in its shape and resists its scattering?
2) Is there Dark matter in interstellar space outside of Galaxies?
3) does Dark matter exist into Galaxies due to gravitational pull of cluster stars which is the main reason of formation of galaxies?
4) if Dark matter exists Out side of the Galaxies How does it let the cluster stars to form galaxies I mean Why Dark matter didn't resist clusters of star to collide into each other to form galaxy?
5) or Dark matter really doesn't play any role in Galaxy?

my comment:

Dark Matter is proposed wherever our models cannot predict something.

For example M31 has a trillion stars. The current model does not correctly predict all those stars in motion. The outliers are at the edges of the spiral arms, which would be at the end of an ellipse. Our Sun is known to not move in an ellipse. Its motion is affected by a few billion stars.

Dark matter is claimed to be in M31 to explain the observed deviation from the model.

Dark matter was first proposed by Swiss astronomer Fritz Zwicky when he observed the Coma Cluster and the high velocity dispersions of the constituent galaxies. He suggested the term dark matter ("dunkle materie"). Harold Babcock in 1937 measured the rotation curve for the Andromeda galaxy and it turned out to be flat, also suggestive of dark matter (or dark gravity). Decades later, in the 1970s and 1980s, Vera Rubin (just recently passed away) and others mapped many rotation curves for galaxies and saw the same behavior.

In each case dark matter is claimed to explain a deviation in motion that is not predicted.
Zwicky thought the galaxies in the Coma Cluster were not moving correctly, though in 1933 I question whether that certainty could be justified. Others followed his lead.
These cases suggest after we improve our understanding of the universe beyond our solar system, including the motion of our own Sun, then dark matter is not required.

Dark matter is the name for 'we are working on that' but unfortunately it is treated as the actual solution, until we learn more.

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