Is dark matter’s “nightmare scenario” true?

In science nothing is provable. Things can be falsified but if a hypothesis is true it can actually never be proven. This is because at any point in time and additional observation can be added to your sample that definitively disproves your hypothesis.

All zebras have stripes. is my hypothesis. I observe 100 zebras and I see they have stripes. I see 10 million zebras and I see they have stripes. There is no amount of observations I can make to prove my hypothesis. However if at any point in time I see a zebra with no stripes, I have disproven my hypothesis.

So anything that is in actuality true, is not falsifiable and also un-provable. In short, some statement about our universe can be true, but we can never absolutely know for sure whether that statement is actually true.

Basically your comment isn't logical because you're saying we shouldn't study anything that is true because it's unfalsifiable by definition.

In addition to falsification, Science can make statements that are vague and UN-definitive. Un-definitive claims include causal claims and correlative claims.

Correlation is A and B are casually linked directly or through some other source. A maybe causes B or B maybe causes A or maybe a C is causing A and B. Causation is A and B are directly linked causally. A causes B.

Science can make the two claims above but such claims are made based off of a sample. If the sample is bad or biased the claim is also bad and biased. Given that we live in an unknown universe of which any sample we extract can only be of finite size out of a potentially infinite size it makes both correlative and causative claims un-definitive. The sample can never fully encompass everything. This is the reason why in science, nothing can be proven.

So in short science can definitively falsify things, and in a vague sense it can make statements about correlations and causation. The later two, though undefinitive can be used to say something about some topic of the universe that is true. But it can't say anything definitive and we can never know for sure.

Because Falsification is limited to things that are false, so much of science doesn't actually use it even though it is a definitive metric.

>As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.

The GP is kind of the one that's not too bright. You're definitely a layman but you ask the right questions. GP is not a layman and he's suppose to know certain things and not say what he said. He's very opinionated and makes broad statements without addressing nuance. So don't worry about it! You're not dumb!

GP is utterly wrong. Falsification is still used on the frontiers of modern physics, though much of it nowadays is just correlative results, but if one such correlation is shown not to hold at all, it becomes a falsification. In fact, EVERY single experiment in modern physics is an attempt at falsification. And when the falsification fails... a correlative result is presented as the final answer.

But note, even a falsification is not clear in certain cases. This is because our scientific observation tools have limited accuracy. Most of the time we just assume the observation tools are 100% accurate. But at the frontiers of modern physics the observation tools are so bad that the tools themselves yield probabilistic answers. So while in science falsification is still very much used to clearly discard theories, in modern physics, a negative result is not indicative of a clear cut falsification.

"Dark matter" implies a new field. Or rather, a new field or force that affects gravity implies some sort of interaction exactly like a massive particle that interacts only gravitationally. Particles are just localized disturbances in fields (in quantum field theory), and new fields can have such disturbances and thus have particles. In this (trivial) sense, MOND is a type of "dark matter" theory that posits a particle with only gravitational interactions.

Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).

I should point out that primordial black holes are one of the candidates for dark matter. The scientific community seems to be flipflopping about the plausibility of this theory.

The key weakness in your comment's parent comment was, "occupy the exact same point".

One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at https://en.wikipedia.org/wiki/Caustic_(optics)>.

Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres?

Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume.

Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres.

One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work.

Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around.

tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.

> If you run LCDM for 13 billion years you don't get galaxies till much later than what we observe via say JWST.

I would not take all the recent claims of extremely high-redshift galaxies, found in initial JWST data, at face value. Several of these claims have already been shown to be false.