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Wikipedia defines

Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe.

And continues with

Various astrophysical observations – including gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen – imply dark matter's presence.

So dark matter is always implied from differences between observed and GR predicted outcomes. In every other field of science we used to call this the textbook definition of error, or in some cases anomaly.

Why did physicists choose instead to justify it with the creation of yet another unknown?

Through three points I can pass a 3-coefficient polynomial. If I'm given a 4th point it will likely be off the curve so I add yet another parameter to fit the 4 points perfectly with a 4-coef polynomial and I can play this game indefinitely.

From an ethical point of view, isn't this immoral?

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    $\begingroup$ Dark matter is an extremely conservative choice: preserve General Relativity and instead assume that there are still more particles yet to be discovered that don’t interact electromagnetically. Why would you think that we have discovered all of them already? Furthermore, I don’t know of any GR replacement that works. $\endgroup$
    – Ghoster
    Commented Nov 7, 2022 at 4:54
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    $\begingroup$ Does this answer your question? Why is "dark matter" theory accepted? Why wasn't general relativity rejected? $\endgroup$
    – John Rennie
    Commented Nov 7, 2022 at 5:26

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From an ethical point of view, isn't this immoral?

Well, that is pretty inflammatory language. In fact, dark matter is not only completely ethical, it is part of the standard and typical progress of the scientific method, and similar things have happened often. I will give three brief examples:

  1. Neptune - when carefully measuring the orbit of Uranus, there were some perturbations that could not be explained through the known laws of Newtonian gravity with the known sources of gravity in the solar system. There was a lot of other evidence supporting Newtonian gravity, so first we assumed the existence of a previously unknown source of gravity with the existing laws. We then tried to confirm it through other means and were eventually successful.

  2. Vulcan - when carefully measuring the orbit of Mercury, there were some perturbations that could not be explained through the known laws of Newtonian gravity with the known sources of gravity in the solar system. There was a lot of other evidence supporting Newtonian gravity, so first we assumed the existence of a previously unknown source of gravity with the existing laws. We then tried to confirm it through other means but were never successful. Eventually, GR was developed (not for this purpose) and was found to explain the perturbation using existing known sources.

  3. Neutrinos - when carefully measuring beta decay, there was some missing energy, momentum, and angular momentum. We assumed the existence of a previously unknown type of particle. We also eventually had to assume that this new class of particles was governed by a new fundamental force. This iterated a bit together with experimental data to eventually close on both a new source, neutrinos, and a new law of physics, the weak interaction.

So here we are again. We have data that doesn’t match the theory with the known sources. We have lots of other observations supporting our theory. So, as we have done many times before, we are considering other sources (called dark matter) as well as new laws of physics (e.g. MOND). Far from being unethical, this is the correct, ethical, and usual approach for dealing with such data.

We do not at this time know if dark matter will be a Neptune, a Vulcan, or a neutrino. My bet would be a neutrino, where we eventually find both a new source and a new law of physics, but that is pure speculation at this point. We simply haven’t reached the end of this path yet. The mere fact that you are impatient to reach the end, doesn’t mean that the people who are forging ahead are unethical.

PS From an ethical point of view we judge things as ethical or unethical, not moral or immoral. Ethics are related to but distinct from morality.

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  • $\begingroup$ +1 And while dark matter has allowed us to make predictions which we have been able to check (i.e. movement of galaxies in galaxy clusters or interactions between colliding galaxy clusters) which means it scales beyond ratation curves of galaxies MOND (which describes many different theories) can in most cases only describe one phenomenon at a time while it has to be modified differently on different scales. $\endgroup$
    – kruemi
    Commented Nov 7, 2022 at 5:45
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    $\begingroup$ And I write this as a person who likened to compare "Dark Matter" to the "Ether Theory" because it looked like a crutch to me too. But after studying a lot of sources I concluded that it's currentl the theory that covers more cases we can see than anything else. But I'm quite sure that we won't find THE DM particle but will have to chip away bit by bit to find all the different sources that make up DM. $\endgroup$
    – kruemi
    Commented Nov 7, 2022 at 5:49
  • $\begingroup$ Is it justified to compare the examples of neptune, vulcan and neutrinos with dark matter/ dark energy in GR? The former examples are not exotic and there were very particular properties which one could look for to validate those hypothesis. For the case of dark matter/dark energy, their nature are completely unknown, the observational data requires them to exist in such a large amount compared to ordinary matter, that it almost appears to be a cheat to enforce validity of GR. So why do we need to introduce completely new parameters which are far fetched from known physics to explain data? $\endgroup$
    – KP99
    Commented Nov 7, 2022 at 6:55
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    $\begingroup$ Neutrinos — particles that can travel through light-years of lead, and trillions of which pass undetected through every square centimeter of our skin every second — were very exotic once. And dark matter, if it exists, may turn out to be no more exotic than neutrinos were. $\endgroup$
    – Ghoster
    Commented Nov 7, 2022 at 7:03
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    $\begingroup$ One should be careful with judging the work of others out of a position of ignorance. "I feel that physicists haven't adequately looked..." --- with all respect, the person that hasn't adequately looked is you. That is fine and what this website is all about, but to judge others in this case is what /I/ call unethical. Neutrinos were completely whacky and appeared unprovable when they were proposed, much in contrast to the expectation of dark matter. Also, as of today, there is not a single alternative theory to explain the data. $\endgroup$
    – rfl
    Commented Nov 7, 2022 at 9:28

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