How does dark matter hold a galaxy together and how do we know where dark matter is more or less dense?

This question might come as a follow-up to my previous article on the subject. This is an excellent set of questions, but, truth is we know little to nothing about dark matter. If we take the entire mass of the Universe, dark matter accounts for around 80%. We don’t know what it is made off, but it’s everywhere, not only in the intergalactic space, but inside galaxies as well. There are groups of dwarf ancient galaxies (like those inside the Perseus galaxy cluster) that remain intact, although the larger surrounding galaxies are torn apart by the gravitational pull of their neighbors. Therefore, we can imagine a scaffolding of dark matter keeping some galaxies in their place with higher densities for stable groups or individual galaxies and lower densities for frivolous ones. 

We have some ways we can measure higher densities of dark matter, like measuring the velocity of some stars inside a galaxy during their orbit around the center of the galaxy. – Roman Alexander

(The question was originally asked by Arnab Khan from India)