In article , Jos
Bergervoet writes:

On 19/12/21 2:53 AM, Phillip Helbig (undress to reply) wrote:
...
No dark matter has been detected.

[apart from its gravitational influence]

... Experiments for direct detection or
production in colliders have turned up nothing. Possibly
self-annihilation is seen in gamma rays at the centre of the Milky Way,
but this is far from clear. There is still a window for WIMPs as dark
matter, probably very low mass (de Broglie wavelengths measured in
kiloparsecs) or high mass (TeV). Joe also thinks, though, that
primordial black holes might be the best bet.

As a theoretical physicist, I actually hope that primordial black holes
do exist, but do not give the answer! (To the missing dark matter I
mean, the universe and the rest put aside for a moment..) Because the
standard model of particle physics would be much more convincing if the
axion were found to exist. Any axion discussions on the symposium?
https://www.quantamagazine.org/why-dark-matter-might-be-axions-20191127/

The axion was mentioned. Someone even showed a picture of Axion
detergent, apparently the source of the name (as it would wash away the
strong CP problem). However, since there has been nothing new regarding
axions (other than more non-detections), there was no detailed
discussion of it. I agree that it is one of the better motivated WIMPs.

Primordial black holes have become more popular for two reasons. First,
an article about three years ago by Carr, K=FChnel, and Sandsted showed
that they are still allowed. Traditionally they had been thought to be
ruled out, but asteroid-mass and several-hundred-solar-mass black holes
are allowed, at least if there is a relatively broad distribution in
masses (which is what one would expect anyway), perhaps even solar-mass
black holes (but not Jupiter-mass).

Mike Hawkins, who was also there, had been strongly advocating
Jupiter-mass black holes, saying that they cause most long-term
variability in quasars, but his arguments have been shown to be wrong.
Some people misinterpreted this to mean that all primordial black holes
as the main component of dark matter had been ruled out. To be fair,
Mike has mellowed a bit, and now says that he got the mass wrong. Mike
gave a talk where he looked at variability in multiply imaged quasars
where at least one image is far from the galaxy. Because they are
multiply imaged, one can use the time delay to get rid of the intrinsic
variability, so what is left is probably microlensing, but at a place
where the density of the galactic halo in stars is way too low.

Another reason is LIGO. The black holes which have been detected so far
are not exactly what one would expect if they were all descended from
massive stars. If LIGO (which is now up to a detection a week) detects
black holes of below a solar mass or so, this will be a strong
indication that they are primordial. The primordial-black-hole scenario
makes some definite LIGO predictions, so that has revived interest in
it. The main thing is that LIGO can rule it out, and since LIGO is
detecting stuff, this is, at the moment, more interesting than WIMP
detectors.

Primordial black holes as dark matter would not require any new physics.
(In fact, it might require new physics to avoid them.)