# The nature of DM

1. Mar 6, 2006

### Garth

There are several pieces of evidence to suggest that DM is a real artifact of the universe.
1. Nearly flat rotation curves of spiral galaxies suggest they are embedded in a massive unseen halo of some kind. (Although the mass of these haloes may be affected by GR gravitational effects and over-estimated by ~30% Over estimate of dark matter).
2. $\gamma$-ray emission from galactic cores may be a marker of DM annihilation.
3. Galactic cluster dynamics likewise require large unseen masses in between the individual galactic haloes. This mass is also detected by gravitational lensing effects.
4. Unseen mass is required to explain the formation of large scale structure early in the universe's history.
5. The observed mass density of the universe, from cluster lensing of distant quasars, seems to be about 30% of the closure density $\Omega_M \sim 0.3$, whereas the visible mass density is only $\Omega_v \sim 0.003$.

The standard model allows a maximum baryon density of around $\Omega_b \sim 0.04$ created by nucleosynthesis in the Big Bang. The WMAP data is consisitent with this and a $\Omega_M = 0.27$, so it requires a component of $\Omega_{DM} \sim 0.23$, and also, if that data is interpreted as evidence of a spatially flat universe, then it also requires Dark energy with $\Omega_{DE} \sim 0.73$.

Note that this standard model still requires a lot of unseen (dark) baryonic matter, the difference between $\Omega_b \sim 0.04$ and $\Omega_v \sim 0.003$, i.e. in the standard model there is an OOM greater amount of unseen baryonic matter than that which is visible as stars and nebulae.

But what is the rest of the DM?

There are numerous alternative proposals including, e.g. Self-Interacting dark matter, Self-Annihilating dark matter, Decaying dark matter, and many others. But a recent paper Cold Dark Matter as Compact Composite Objects suggests
The paper then explains how this might have happened in the "color superconducting phase" of the BBN and that today DM particles are strongly interacting composite macroscopically large objects which made of well known light quarks (or even antiquarks).

Whatever the merits of this model, the interesting idea from my POV is the concept that a link between the nature of DM and baryonic matter may resolve some problems with the standard mainstream model. IMHO this DM is baryonic in nature.

Garth

Last edited: Mar 6, 2006