Caption: "Projected density plot of a cosmological redshift z = 2.5 dark halo from a cosmological N-body simulation. The visible part of the galaxy (NOT shown in the image) lies at the dense center of the dark halo and has a diameter of roughly 20 kiloparsecs. There are also many satellite galaxies, each with its own subhalo which is visible as a region of high dark matter density in the image." (Slightly edited.)

The image is false color---but you knew that, right?

Dark halos:

1. Almost all galaxies seem be embedded in dark matter halos which are of order 10 times more massive than the baryonic matter: ordinary matter (i.e., protons, neutrons, and electrons) that make up stars, interstellar medium (ISM), and sometimes intracluster medium and intergalactic medium (IGM).

2. The dark matter halos are believed to be roughly spherically symmetric and their centers are approximately the centers of the galaxies. The dark matter probably extends well beyond the observable galaxies.

3. The dark halos are the main source of gravity that led to the formation of the galaxies as we know them.

   The dark matter itself is believed to consist of a swarm of exotic particles whose nature we do NOT yet know.

   Almost the only effect of the exotic particles is through their gravity.

   They are extremely unreactive via other fundamental forces with themselves or anything else.

   The dark matter particles form a kind of gas, but NOT an ordinary gas since they barely interact.

   They just move around in chaotic orbits which is why they can be modeled by N-body simulations.

4. The dark matter particles are cold dark matter: i.e., nonrelativistic matter.

   If they were hot dark matter (i.e., relativistic matter), they would NOT have clump together to have helped form galaxies.

   Cosmic neutrinos form at least a part of the hot dark matter of the observable universe, but at present they seem to have little effect on cosmic evolution.

5. There are many theories about what exactly the dark matter (i.e., cold dark matter) is, but no consensus theory so far.

6. Experimental searches for dark matter are ongoing and keep giving tantalizing hints of positive results without ever firming up it seems.

7. But maybe there is NO dark matter particles making up cold dark matter.

   Maybe the cold dark matter consists of primordial black holes.