mgb_phys said:
You can work out how much a galaxies 'weighs' by looking at it's rotation speed, if you then add up all the stars in the galaxy you only get a few % of this mass. This 'missing mass' is dark matter - called dark because it isn't bright enough to be seen in a telescope.
There are two theories:
Cold Dark Matter (CDM also known as MACHOs) says that it is just ordinary lumps of rock and gas which aren't big enough to form stars and so don't emit light.
Hot Dark matter (also known as WIMPs) says that it is new sub-atomic particles such as super massive neutrinos which are rare enough on Earth that we haven't seen them.
Sorry not my field so I don't know which theory is winning. Wikipedia has quite a good page on this.
"Sorry not my field" -
We can measure the relative abundance of different components of the mass and energy in the universe by \Omega in comparison with the critical density, which is given by:
\frac{3H_0^2}{8\pi G} ~ 10
-29gms/cc.
So this density is given as \Omega = 1.
The maximum density allowed by Big Bang Nucleosynthesis (BBN) in the standard model is \Omega_b = 0.04.
However the maximum density observed as visible stars and bright nebulae is \Omega_v = 0.003
When the rotation curves of spiral galaxies, the velocities of galaxies within galactic clusters and the lensing of distant quasars by closer line-of-sight clusters the density required for all three, together with the WMAP data and the requirement for large scale structure to form, turns out to be \Omega_m = 0.27.
Thus there is a lot of baryonic matter (ordinary hydrogen etc.) that cannot be seen, and a larger component of mass that cannot be seen that cannot be baryonic, \Omega_{DM} = 0.23.
It is this exotic non-baryonic matter that is normally referred to as DM, the only problem being it has not yet been found in 'the laboratory' even after three decades of looking. The particles of DM could be only slowly moving relative to ordinary galaxies and such, in which case it is called Cold Dark Matter CDM. Or it could be moving relativistically relative to the visible stuff, in which case it is called Hot Dark Matter.
The present model generally makes use of CDM.
The ring of DM recently discovered is observed by analysing the gravitational lensing of the many distant sources that lie behind it. It seems to have been left behind after the two galaxies at its centre collided,with their DM halos, having traveled along the line-of-sight.
Garth
