Is it implied in "I don't understand" that "it's not understandable?" There is quite a bit of evidence that point towards dark matter and away from problems in our theories of gravity. We have direct evidence for forms of matter that do not interact with particular fields, it would be unsurprising to find other forms that are extremely difficult or impossible to detect.
At the galactic level, however, there is a lot of cases where it seems you can see the mass distribution of dark matter. They've found starless hydrogen clouds that seem to be dragged around by a dark matter halo.
If you think about the evidence from galactic rotation curves you are likely to think that "this galaxy has some dark matter in it" but the modern point of view (which seems to work) is that "this dark matter has a galaxy in it."
When it comes to cosmology at the larger scales I don't really believe in the "multiple flavors of dark matter and dark energy" that is fashionable now. I wonder, for one thing, if the universe is really homogeneous at large scales and if that breaks the assumptions of current models.
https://royalsocietypublishing.org/doi/abs/10.1098/rsta.2011...
Part of why dark matter is such an attractive explanation is because there are no constraints on it. Unfortunately this seems to have relieved pressure to explore and test modified gravity.
But you are correct, I don't know what I am talking about which is why my comments were phrased in the form of a question or literally with the words "I don't understand"
Something like this?
https://chandra.harvard.edu/xray_astro/dark_energy/chart.jpg
With Neutrinos we know how they are created and have a very good upper limit on how much there should be and it's not nearly enough to explain the observed effects. Same for the CMBR.
I'm not saying there couldn't be much more Dark matter/energy than neutrinos or photons but it's a bit too convenient to introduce a variable that is allowed to take any value and match it to observations without an explanation of what it is (besides having mass) or how it is created.
We won’t find dark matter in your or my lifetime, because it doesn’t exist. It is a convenient, flexible, and inelegant fudge factor.
If there's really just modified gravity without any dark matter, the distribution of the regular matter would be sufficient to model the gravitational dynamics but if there is such a thing as dark matter, you'd see that the gravitational effects would be consistent with a center of mass which is displaced from the center of mass for the regular matter. This is, in fact, what you see, suggesting that there really is some type of dark matter.
From observations, we can see dark matter acting independently of visible matter. It's very difficult to explain this any other way.
The alternative theories don't fit the entire set of data as well. Or they do, but by choosing additional complexity, rules, constants that are chosen to make the model work, but can't be explained otherwise.
Dark matter isn't a full explanation, no doubt, but imho it's the best we have.
We see an effect which isn’t accounted for by the things we understand well and come up with several theories to explain that effect. Eventually we gather enough evidence to confirm or deny those theories and science marches on.
There are several theories as to what causes the effects that are primarily attributed to dark matter, by no means is it settled. But the theory that fits the best is that there is quite a lot of mass out there that we can only observe so far by large scale gravitational effects on matter we can see more easily.
Sure, it could be something else but a really convincing candidate hasn’t come up.
We’re in the same situation physics was in towards the end of the 19th century. It seems like physics is nearly “done” with only a handful of odds and ends left unexplained. Maybe it is, maybe we’ll get a breakthrough that opens up a whole new world of physics. It’s hard to be sure but over and over we keep probing and not really finding significant “new physics”.
~50 years on without significant progress for particle physics + GR means we are starting to be interested in alternatives.
Theoretical particle physics is even worse. There haven't been any advances since the 1970's. Experimental particle physics has done a great job in verifying/testing the predictions from the 1960's/1970's but theoretical physicists are stuck in a rut.