In a rush to tailor the evidence to a flawed theory, dark mentor was invented by humon minds in an attempt to save a beloved theory. We need to cast off the shackles of what we want to be true, and look at the evidence in a cold, anyalytical light. When this is done, I'm quite certain that there will be no need for the magical fairy dust matter that is there but isn't there.
The term dark matter is just the name for a discrepancy. For example, the galaxy rotation speed is 220 km/s at our position in the galaxy (8kpc), and stays the same until 30kpc. But the number of stars, which are the mass we can see, declines exponentially. So some mass (10x more than what we see) must be there to keep the rotation fast (otherwise it would be like the solar system -- Pluto rotating around the sun much slower than Mars).
Then in clusters we see that gravitational light acts as a lens and we can infer the mass that bends the light behind it. And it is much more than we see in stars and gas.
In the cosmic microwave background, which is a relic from the last time electrons and photons interacted very strongly, 380000 years after the "Big Bang", we can estimate the density of the universe there. Also, the fraction of matter interacting with photons, is only a fraction of the total matter there.
All of these *different, independent* probes, and several others, point to the same ratio of total matter to electromagnetically-interacting matter.
Now you can take the state of the Universe at 380000 years age, with its total matter, electromagnetically-interacting matter and photon budget and evolve it following general relativity. And people find that the clustering of galaxies, their total number and sizes can be reproduced quite well. And this is not possible without putting that additional, non-electromagnetically-interacting matter there. And In this experiment you can learn something about how weak the electromagnetic interaction must be (for example, a large population of Neutrinos can be excluded, because they interact to strongly, smoothing out the structures).
As you say, another path is to modify the theories of GR, and every week there are papers explaining Dark Matter with alternative theories, sometimes in combination with Dark Energy. This is a path that many people are working on. If you see the term "Dark Matter" as the *name of the problem*, namely the discrepancy between observations and normal matter + GR, then there is no conflict, it does not say how to solve it. Dark matter is real, because the discrepancy exists. And the search for particles is also not concluded yet: Larger, cold objects have been proposed (e.g. brown dwarfes, Jupiter-size planets), as well as new fundamental particles (Neutrinos, as well as as-of-yet unobserved particles, like the sterile Neutrino, or totally new particles from some theories of supersymmetry). Some of them have been excluded -- for example it can not be stellar-size black holes, because of the number of binary star systems we observe in the outer parts of the Milky Way; those would be destroyed by frequent interactions with a large population of masses. The upgraded LHC will try to produce more particles, and there is a real chance it will produce (or exclude) a specific candidate dark matter particle predicted (proposed) by supersymmetry.
Believe me, Astronomers really do not like the idea of Dark Matter, and have been fighting it for decades. But the evidence from many different experiments is there. We still don't know what it is, whether the laws have to be changed or additional particles have to be put there (and which ones). But the range of possibilities is getting smaller and smaller. And putting particles there that do not interact except for gravity has been very successful in explaining various observations. I used to be cautious because in principle you could just arbitrarily put mass where you need it -- but if you start from the Big Bang and only use general relativity, then the created galaxies with dark matter in/around them, or galaxy collisions like the one in this article just come out -- there is no choice involved here except for the density of dark matter in the early universe.
Many different observations, proposed resolutions in new theories, proposed particles and detection experiments are listed on the Wikipedia page https://en.wikipedia.org/wiki/...
For this particular observation, you should note that they observed 60 collisions, which act like we think dark matter acts (no collisions, only gravity), and one is odd. That should tell you to be cautious, perhaps something is peculiar about this system or the observation.
