[madbadgalaxyman (Robert)]

I once asked a leading professional authority on dark matter this question: how certain are you that the dark "material" actually exists?

He said to me that, while there may eventually turn out be other explanations for why a low luminosity dwarf galaxy holds together (that is, does not dissipate) despite it containing just a smattering of widely spaced stars, we have to go where the data leads, and adopt the simplest explanation;
which is that there is additional gravity being provided by actual physical matter in some unknown form.

Low luminosity dwarf galaxies have a very low spatial density of their constituent stars, but the total matter density of these galaxies, inferred from the amount of gravitational force that is necessary to hold them together, makes them sort of "like cannonballs".
It is a while since I read papers on dwarf galaxies, but I seem to recall that the smallest dwarf galaxies, in order to remain stable under the influence of their own gravity, must contain several hundred times as much "dark" matter as "luminous matter"(luminous matter is matter that is evident in the various wavelength bands that we use to observe a galaxy).

Similarly, given that some galaxies can move around within clusters of galaxies at orbital speeds of ~1000 or even ~2000 km/s, it is perfectly logical to believe that these galaxy clusters could not exist for long without extra gravity (or some other force) coming from within such a cluster. In other words, there is gravity in excess of that coming from the observed constituents of the cluster.

Of course it is possible to be cynical about cosmologists who seem to pull large quantities of dark matter "out of a hat" in order to explain how the existing cosmic structures can form in the requisite amount of time....that is, the additional "dark" matter has to be there for gravity to be able to complete its structure-forming work in the 13 billion years available to form the observed structures in our universe. Indeed, this kind of assumption does seem to be "cooking the books" to try to make the existing cosmic structures as we now observe them.
For instance, is the largest-scale cosmic structure of voids, walls, and bubbles, something that is naturally formed by gravity alone? True, cosmologists can "make" these large-scale structures in only 13 billion years using only the gravitational force and large quantities of non-luminous matter in some unknown form, at least in their computer simulations, but "universe in a computer" does not have to correspond with the real universe.

For the moment, however, "these are the truths that we must cling to".......the most logical inference, for now, is that a galaxy cluster or a galaxy will come apart if there is no additional gravity coming from some form of unseen matter. We can, of course, engage in informed speculation about unknown forces and suchlike that might influence stars and galaxies, but for the moment we must believe at the 70% level of certainty that dark matter really exists. While dark matter is not exactly a fact, we must concede that it is probably there!
(cosmologists are great ones for assigning 100% certainty to their theories....but certainty is the domain of theology, not science.)