Well, I think the only way to explain an anomalously large annihilation signal from dark matter is to assume that we actually just happen to be in an unusual region of the galaxy with a good amount of extra dark matter nearby. So I think the more likely explanation is that any such signals are just due to some poorly-understood astrophysical process.

Ok, that’s exactly the point, what about the bounds about collisionless DM versus the pamela results that require large cross sections (Somerfeld enhanced?). We are not in a particularly hot spot in the galaxy, so this is happening everywhere, how can it remain ‘colisionless’? can you throw out some numbers?

Must CDM be self-collisionless or could it simply be
reluctant to form small scale dark matter structures?

I’m pretty sure those are one and the same question. In order to collapse, it must emit energy in some way. And in order to do that, it has to collide with things. Obviously there are some limits as to just how collisionless it must be based upon our current observations of dark matter both in the early universe and today. For the possibility of a dark charge, you may want to check out Sean’s relatively recent blog post on dark photons.

A large pool of dark matter would not only slowly accrete
more dark matter, it would also sweep up normal matter,
which has probably been denser at small scales than DM
since BBNS. The denser material would sink into the pool.
If DM is allowed to become too dense, the normal matter
simply would sink into the centre of mass forming layers
of increasingly heavy atoms and nuclei. In the early days
of initial structure formation, that would mean isotopes
of H to Be and the products of huge pressures upon the
corewards layers (heavier elements, and heavy isotopes
that are stabilized by pressure).

Well, not quite. Big bang nucleosynthesis (BBNS) is pretty insensitive to the nature of dark matter as the universe just had hardly clumped at all back then. BBNS was driven almost entirely by the overall temperature and density, not the minuscule fluctuations thereof. What places the most stringent limits upon dark matter’s properties in the early universe is the CMB itself, which basically places an upper limit upon the temperature of dark matter (too warm, and you don’t get any structure, but it can be as cold as you like).

Could the annihilation question also be put as: if a dark
matter structure arose in a part of the universe with a
very small amount of visible matter, would it eventually
accrete sufficient dark mass to trigger ongoing
annihilations in the core of what by analogy could be
called a “dark matter star”?

Not sure. Depends upon the details of how quickly it loses energy to its environment. For example, if annihilations are sufficiently common that more energy is lost through annihilations than collisions, I suspect that the dark matter halo may never gain in density significantly except by the addition of new matter, such that it won’t ever achieve the densities required. If the collisions are common enough that they dump more energy into their environment than annihilations, then sure, I might imagine something like this happening, but it’d take a very long time.

Sure, but not at very high densities in wells like our
solar system, or surely we would have noticed it affecting
planetary orbits and the sun’s physics.

Right, the effect is pretty small, and you need very large amounts of normal matter for the overdensity of normal matter to count significantly. And yes, the converse is also true, that normal matter is more likely to be found in the more dense regions of dark matter. But the point is that the normal matter interacts much more strongly and tends to fall inward into the potential well, whereas the dark matter stays relatively stable.

Thanks. I have a couple dumb, unrigorously framed questions:

Right, the dark matter doesn’t collapse much
on its own, because it is almost collisionless. Once you
get a halo of dark matter, unless it interacts with
something external to it, it largely just stays as it is
for then after.

A DM halo obviously interacts with lots of things external
to it, at least gravitationally: the rest of the mass in
the structure the halo is part of, and mass external to
the structure.

Must CDM be self-collisionless or could it simply be
reluctant to form small scale dark matter structures?
Obviously it is much less dense than normal matter,
because it went unnoticed by classical astronomers like
Kepler and Newton (and pretty much everyone else until the
20th century). Perhaps there is a dark charge which works
against gravitational attraction analogously to electron
degeneracy pressure.

dark matter is still more likely to be found
in wells where normal matter has condensed into compact
objects

A large pool of dark matter would not only slowly accrete
more dark matter, it would also sweep up normal matter,
which has probably been denser at small scales than DM
since BBNS. The denser material would sink into the pool.
If DM is allowed to become too dense, the normal matter
simply would sink into the centre of mass forming layers
of increasingly heavy atoms and nuclei. In the early days
of initial structure formation, that would mean isotopes
of H to Be and the products of huge pressures upon the
corewards layers (heavier elements, and heavy isotopes
that are stabilized by pressure). Conversely, if DM is
forbidden from becoming dense enough, we have something
similar to the hot dark matter smearing problem of
attracting mass out of the centre of galaxy scale
structures. However at a “Goldilocks” temperature, the
outer layers of the structure would retain a lot of mass,
counteracting the infall of mass to the centre of the
overall structure, and would thereby enable the formation
of stable stars and star clusters.

This is why, for example, there have been some
suggestions that perhaps there’s some annihilation of dark
matter going on near the galactic core.

Could the annihilation question also be put as: if a dark
matter structure arose in a part of the universe with a
very small amount of visible matter, would it eventually
accrete sufficient dark mass to trigger ongoing
annihilations in the core of what by analogy could be
called a “dark matter star”?

dark matter is still more likely to be found
in wells where normal matter has condensed into compact
objects

Sure, but not at very high densities in wells like our
solar system, or surely we would have noticed it affecting
planetary orbits and the sun’s physics. There may be a
gentle density gradient within the solar system, but there
are tight limits on that from observations of
trans-Neptunian objects, right?

So, couldn’t one also say that hot, dense normal matter is
more likely to be found in wells where dark matter has
condensed into relatively dense structures?