Positing some mystery matter that doesn't behave like anything we've observed strikes me as more absurd than gravity working differently at galactic scales, so I guess it's all a matter of perspective.
It is more absurd. Firstly, WIMPs do not "behave unlike anything we've observed". They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds. No new fundamental forces are required to explain WIMPs, they are simply particles we haven't discovered yet. Given that in the 20th century alone we discovered photons, protons, neutrons, anti-particles, quarks (and their whole family of composite particles in the form of baryons and mesons), neutrinos, and weak bosons while in the 21st century we've discovered the Higgs it would be ridiculous to say that new particle discoveries are beyond the realm of possibility. Moreover, we know that the standard model of particle physics is incomplete due to the existence of neutrino oscillations (among other things). So we can be fairly confident there are new particles yet to be discovered, and we have a model of dark matter which fits a hugely diverse set of data fairly well that no other model fits. To deny that dark matter is the best model for the data at present is nothing more than superstition.
I have a engineering-physics degree, and while I will admit my day job involves more down to earth physics software simulation rather than advanced cosmology and particle theory, I would categorize WIMPS more so as the model with the least amount of gotcha's when compared with alternative gravity theories. I for one lean towards alternative gravity theories.
"They behave similarly to neutrinos except are much more massive..." And yet we can build neutrino detectors and "see" neutrinos. So far similar detectors for dark matters have failed to detect anything:
Lack of detection, plus this sparse galaxy with no sign of dark matter, is starting to slowly rule out the simpler explanations by virtue of outliers that don't fit any model.
You yourself mentioned it, but I always remind detractors that the standard model IS WRONG.
The sparseness of this galaxy is what intrigues me. What if the lack singularity at the center is the key?
> The sparseness of this galaxy is what intrigues me.
But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball.
> They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds
You've mixed this up a bit.
Because neutrinos have such a low invariant mass, they have little inertia, so are readily accelerated to speeds comparable to the speed of light at production time or when scattering off an atomic nucleus. A heavy neutrino has a higher invariant mass by definition, and thus more inertia, and thus are likely to move less than a lower-inertia regular neutrino when encountering an an atomic nucleus.
Since there is a lot of gas and dust around at the start of structure formation, you need some mechanism to keep energy-momentum localized around the matter that will become luminous matter like stars and hot gas. Adding inertia to a particle that is otherwise highly comparable to a neutrino would do the job. Normal neutrinos approaching close enough ("scattering") to an atomic nucleus would tend to get a large kick, which can be considered as a substantial Lorentz boost. Heavy neutrinos suffer a smaller kick.
There are is an additionally point worth considering here: there are almost certainly cold regular neutrinos in a relic field called the cosmic neutrino background, which is analogous to the cosmic microwave background. These neutrinos rarely interact with atomic nuclei, but when they do they are liable to get a big kick. They also are so low-energy that there would have to be an enormous amount of them if they were a major component of Cold Dark Matter; enough that there would be visible nuclear-reaction signatures in our sky as they get heated up by collision with hot gas and the like in the galaxies around which the standard cosmology expects there to be lots of Cold Dark Matter. The heating up of such huge densities of cold standard neutrinos (by weak interactions with hot baryons) would also kick many of the neutrinos out of galaxies over time, which produces a smearing out of visible matter as an observable.
Those galaxy-scale observables and the peaks in the CMB power spectrum (plotting the fluctuations in the CMB temperature spectrum at different angular scales) preclude primordial cold standard neutrinos as an important component of Cold Dark Matter operating since the formation of the earliest galaxies.
No, the rest energy of a neutrino is always less than that of a heavy neutrino, by definition. A heavy neutrino can certainly move relativistically; a neutrino likewise can move non-relativistically. The cosmic neutrino background is a huge number of non-relativistic, i.e. thermal, neutrinos.
So, "... except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds ..." is wrong on two fronts, and "... [heavy neutrinos] travel at much slower speeds" also will not be true if they feel the weak force, and if they don't it will still likely be true for heavy neutrinos that find themselves near a sufficiently dramatic event like a highly asymmetrical star-degenerate white dwarf supernova.
No, you didn't, you said "energy", which is an observer-dependent quantity. "Rest energy" is on the other hand Lorentz-invariant.
You can always find some observer which will see even a chosen massless particle as having a lot more energy than a chosen massive particle, although then you can find observers who will see the opposite. The invariant quantity is what matters.
A comoving observer with much better detector technology than we have today will see relativistic neutrinos and thermal neutrinos in the comoving frame, and (if they exist) relativistic heavy neutrinos and thermal heavy neutrinos in the comoving frame. If cold dark matter is mostly heavy neutrinos, then the energy density measured at a typical point in the comoving frame will have thermal heavy neutrinos as the largest component of these four.
However, nature doesn't single out comoving observers' measurements as more real than anyone else's, the real universe has overdensities (there are relativsitic neutrino jets produced here on Earth, for instance) and underdensities, and the motion of even planetary bodies might kick thermal neutrinos and thermal heavy neutrinos up to relativistic speeds, or conversely wrench relativistic ones down to speeds comparable to Earth's orbital speed (in e.g. a solar system barycentric frame). You would have to know at least two not-yet-known parameters for heavy neutrinos in order to say whether (still in a solar system barycentric frame) Earth would kick a CNB neutrino from thermal to relativistic speeds with greater probability than a CDM heavy neutrino from thermal to relativistic speeds.
But, justifying a paragraph I wrote a bit earlier, in a lab frame with a neutrino or heavy neutrino held at rest at the origin of a set of coordinates when we elastically collide the particle with a heavy atomic nucleus (thus fixing interaction cross section), the former is more likely to wind up moving away from the origin at relativistic speeds than the latter, thanks to its rest mass. I had underspecified the coordinate conditions.
Now you're just being needlessly pedantic. I wasn't writing a paper, I was making a comment aimed at the public at large. There's no confusion about "energy" in that context, the observer is obvious, the reference frame is obvious.
Also, I made no mention of a "heavy neutrino", again you keep reading beyond and misinterpreting my words for no good reason other than to jump into a thread and shout "well, actually!" Which is not helpful towards educating the lay public on the matter whatsoever.
