michaelrccurtis

For anyone interested in why there's a lot more to it than this argument, the first equation:

Mg = C1 * Rg * Rg

Is not a good assumption for the discs we observe in the universe, because they do not have a constant surface density. The picture is further complicated by the fact the many disc galaxies also contain a central bulge, large enough to effect the potential (to say nothing of Dark Matter).

I really encourage you if you are actually interested in the arguments to progress beyond the entry level explanations. Binney & Tremaine (https://press.princeton.edu/titles/8697.html) is the standard work - I highly recommend it. At the very least, it will put to rest your notion that astrophysicists only consider spherically symmetric potentials or fail to consider the effects of a disks etc., which is just patently untrue.

It always depresses me (as an astrophysicist) how poor the communication is on dark matter, and this leads people to unfair conclusions. We have known for some time about the limitations of our understanding of gravitational dynamics in extreme environments - the issue is not that "our theories must be right", it's that we have to square General Relativity (which is phenomenally well tested in other regimes) with the results we see. There are broadly 2 different ways of doing this: you can postulate that not all particles interact with electromagnetic radiation (after all, why should they?) or you can come how add extra terms into your gravitational laws.

Both of these have been tried, but there are many reasons why the majority of astrophysicists prefer the dark matter approach. Firstly, there are observations which suggest large regions of mass that are not aligned with the visible mass (e.g. the bullet cluster). Secondly, dark matter can explain the statistical properties of the Cosmic Microwave Background radiation in a way that can not be achieved with ordinary matter alone. Thirdly, when we include dark matter in simulations, we get results that are a very good match for the universe as we observe it. etc. etc. If that weren't enough, we know that there are plenty of types of matter that are dark - e.g. neutrinos, early black holes etc. Whilst none of these have yet satisfied the conditions for dark matter as we understand it, it's perfectly possible that we have already identified some of the constituent parts, and there is a list of potential candidates as long as my arm that the particle physicists are working their way through.

Finally, the biggest annoyance is the intimation that we are all just one big cabal. Let me put it like this: if you could provide indisputable evidence that dark matter was wrong, you would be a superstar in our field and everyone would be phenomenally excited, because it would demand the hunt for new physics. Scientists are a rational bunch, however, so any evidence must be weighed against the existing evidence already observed (which, as above, is substantial).

Dark energy is slightly different. No one know's what this is, but models with a dark energy component do very well at modelling the universe. The question is, why? No one is pretending that this is a solved question, but what we do know is that whatever is driving the universe's accelerated expansion has properties that are very similar to what dark energy is.

tldr: There are loads of great reasons for thinking dark matter exists, contrary to many posters here, and it is well accepted that dark energy is a mathematical term that represents something of unknown physical origin.