“ All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. ”

There is also the galactic rotation curve evidence. With Newtonian physics, the visible mass in galaxies should rotate faster toward the center and slower toward the edge. This is the rotation curve. We actually observe a very linear curve, where the outer stars and gases rotate at the same speed or even faster than the centers. A dark matter halo would provide the additional gravitational mass for cohesion.

>All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. I see no mention on how this very convoluted model can reproduce it.

I am not an astronomer, but IIUC (and I may not), the first evidence for dark matter was posited by Fritz Zwicky[0] in 1933 based on the rotational velocities of galaxies, work by Vera Rubin[1] confirmed Zwicky's hypothesis in more detail decades later.

Since then Rubin's work has repeatedly been confirmed.

And while Penzius and Wilson "discovered" the Cosmic Microwave Background (CMB) a few years before Rubin published her data, (again, IIUC) the CMB was not used as a reliable tool to look for dark matter until better quality data was gathered in the 1980s and 1990s.

[0] https://en.wikipedia.org/wiki/Fritz_Zwicky#Dark_matter

[1] https://en.wikipedia.org/wiki/Vera_Rubin#Rotational_curves

Edit: Clarified prose.

The neutrino was also a particle that nobody could observe at the time when Pauli proposed it. It was just a manifestation of the energy and spin conservation.

The Higgs is the same. It is not needed, but it solves the mass problem of the weak force. It is the only scalar field so far that we have observed and it was not clear whether it would exist at all.

Quintessence and sterile neutrinos are also just pieces that make the equations of the world look prettier, but they are also candidates for dark energy and dark matter.

Pauli wrote in his famous letter:

"I agree that my remedy could seem incredible because one should have seen those neutrons very earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honoured predecessor, Mr Debye, who told me recently in Bruxelles: “Oh, It’s well better not to think to this at all, like new taxes”. From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge."

https://icecube.wisc.edu/neutrino-history/1931/01/1931-pauli...

Well, the point is actually pretty simple. You start with an equation that works for some galaxies. Then you find a galaxy where it doesn't work - you adjust the equation and now it works again. But then you find a new galaxy where it still doesn't work, you adjust again. You try this for 5, 10, 20 galaxies, just adjust the equation - but at some point there is a limit where the equation doesn't bare any more adjustment, and you still have hundreds of other galaxies that don't work.

So, the alternative that starts being simpler is a single simple equation that works for all galaxies, but allow each galaxy to have varying amounts of stuff in it with mass, but that doesn't interact electromagnetically. Right now, this is the simplest solution we have that fits all observations well.

Is it the right answer? We won't be sure unless we can detect particles that fit the necessary characteristics, and a theory that explains the distribution of these particles in different kinds of galaxies. Unfortunately, the models we have allow these particles to be arbitrarily hard to detect, at the level that we can't really rule them out even if we had a particle accelerator the size of the Earth that didn't find them.

Now, in principle a different equation could exist that has the same solutions as the current equations where they work, and different solutions where they don't work, without adding O(number of galaxies) extra parameters. But just like the dark matter particles, unless we stumble upon it, we can't know if it exists or not.

It doesn't, and the problems have only become more problematic over time, but it's the least bad hypothesis that's broadly accepted. I suspect a generational succession is required for new paradigms to be contemplated.

There are many researchers proposing simpler, novel, and testable solutions that seem to go unnoticed. For example, I'm a fan of Alexandre Deur's work. He has some simple and elegant solutions that I've never seen discussed even though they appear "obvious". For example, from 21 years ago: https://arxiv.org/pdf/2004.05905

That paper is suggesting that one of the reasons why galaxies are spinning faster than some calculations expect is because they're failing to account for the gravitational lensing of gravity itself, which bends gravity down towards the disk.

This is basically the first and most common objection laypeople have. (In fact, this comes up in every single thread on HN about DM, not exaggerating.) Believe it or not, scientists have thought of that as well. They tried, and nothing fits the data as well as the standard model.

The trope is so common that there even is an xkcd for it:

https://www.xkcd.com/1758/

> never really understood how just adding extra invisible stuff to make the equations work is justified

IANAP but here’s my understanding.

At the end of the month you spent $2000, you’re not sure how so you track down your expenses:

  - rent $500
  - groceries $120
  - gas $80
  - …
  - unknown: $123

Or maybe there's nothing wrong with the equations, they're just the wrong equations. Or based on a fundamentally wrong assumption.

> Proponents of all of the above are looking for a grand unifying cosmology. Dark matter and dark energy are confusing and unknown, so people want to just simplify them away. Flat earthers want to simplify away the existence of other planets. Antivaxers want to simplify away medicine.

Ironically, you are positing a grand unifying theory here by stating that detractors from mainstream opinions just want simpler answers. But isn't it simpler to just go with the mainstream opinion and go with the flow? Alternative models of the earth are typically rooted in Biblical or ancient belief systems. Any specific argument based on physics come long after a person's acceptance of, e.g., the Bible. And I can't even imagine how you might think anti-vax beliefs are simpler so I can't speak to that. Homeopathy sounds pretty dumb to me, sure, but also there are tons of reasons to distrust the medical system in the USA, I'd be surprised if anyone seriously argued against that. I can understand why people suspicious of America's medical culture of prescribing 10 medications to someone instead of lifestyle changes might see a bottle of sugar pills at the store labeled with "natural remedy" and think it could be a better alternative.

> Every new observation has completely overturned the predictions of every MOND or modified gravity theory, and they just come back with a totally different explanation to fit the new data.

That's how science works. Scientists are not prophets, they do not have the luxury of starting with all the answers. It's really disheartening to see you make these theories into an "us vs them" ego contest.

I'm guess I'm just a dyed-in-the-wool agnostic, but when I hear someone suggest an alternative theory I don't immediately feel the need to group them with everything I think is wrong with the world. Instead I think, "That's really cool they came up with a different model of the universe. I wonder how they account for everything we observe and how it will be refined over time and whether it will predict new discoveries." Even if your current theory never gets overturned, I think your attitude is a perfect example of why science is said to advance one funeral at a time. Just because they got it wrong in the past doesn't mean they can never get it right, and it doesn't mean there's a 100% certainty that you got it right.

"There are- things like flat or hollow earth, antivax and HIV deniers, homeopaths etc."

That's a fair point. It's just my perception that those things don't tend to be taken very seriously by people commenting on HN.

What massive gaps?

Rapid expansion of the early Universe. We need to have Inflation in order for the Big Bang to work as a theory.

Without it, the Big Bang theory runs into major problems:

1. The Horizon Problem: The cosmic microwave background (CMB) has nearly identical temperature in all directions, suggesting the entire observable universe was once in contact. But without inflation, opposite sides of our universe would never have been able to “communicate” and reach this equilibrium.

2. The Flatness Problem: Our universe is remarkably “flat” (parallel lines stay parallel), which would require impossibly precise initial conditions without inflation.

3. The Origin of Structure: Inflation explains how quantum fluctuations during this expansion became the seeds for galaxies and cosmic structures we see today.

It's obviously tied pretty closely to any theory of large scale structure formation too