Over in print media today, New Scientist's cover article was on a "new" idea for what dark matter is. Apparently, dark matter should be composed of Dark Atoms, in a theory introduced by Christopher Wells, et al. In fact, astronomers and particle physicists have been studying many ideas of what dark matter could be for quite a while. Occasionally a summary of the current ideas gets published, such a the 2004 classic, by Silk, even then half the current ideas get left out, such as CHAMP, mirror matter, Q-balls, super weak dark matter and many others.

Still, I have to wonder if researchers get the time to read the literature because the idea of forming dark matter out of dark atoms is hardly new. With the mirror matter theory, fifty plus papers have been published, notably by Robert Foot, and also by Paolo Ciarcelluti who wrote his PhD thesis on it. Mirror Matter is a complete copy of ordinary matter, except invisible (it doesn't react to our light, only its own mirror or dark photons), complete with mirror atoms and potentially stars.

    When it comes to dark matter, the simplest thing it could be made of is particles with mass that don't interact much, WIMPs (weakly interacting massive particles). WIMPs are popular with supersymmetry theorists, because super-symmetry can lead to a stable massive particle, provide that the introduce an ad-hoc symmetry to keep the proton stable, called R-symmetry, which unfortunately has to be broken somehow.  Recently its been looking like WIMPs acting as cold dark matter don't form a universe that looks like ours. They clump to much in the centers of galaxies and globular clusters, and they seem to form to many dwarf galaxies. 

  Perhaps the next simplest thing a researcher might do is take the known particles and form a complete set of invisible copies of them. This is what mirror matter does, and at the same time mirror matter restores the symmetry between left handed and right handed particles, parity, which is broken by the weak nuclear force. In Mirror Matter a symmetric universe is made has is a left handed weak force and a preponderance  matter particles in one half, and a right handed weak force and a preponderance of mirror anti-matter particles in the other half.

   Could we solve dark matter with a simpler dark side?. It can't just be composed of dark electrons and a dark photons, in that simple model the plasma in galaxies doesn't help build structure at all.

Chistopher Wells' model is a little simpler than mirror matter, because although he has dark electrons and dark protons, he doesn't seem to have any dark neutrons, meaning no dark stars or supernova. Robert Foot on the other hand thinks we need mirror supernova to keep the mirror matter hot enough not to collapse into the center of the galaxies, and as a plus these mirror supernova would also generates a positron excess as observed by the PAMELA satellite. Dark Atoms would form dark brown dwarfs and black holes but not normal stars, could we see this difference in the dark content of galaxies,  potentially the amount of matter forming MACHOs, dark compact objects in the outskirts of galaxies would be different, and without the dark star burning these object would be much more compact. They would still be invisible because dark atoms or mirror matter don't emit ordinary light, only they mirror or dark side version of light.

   Hope for actually finding dark matter in experiment might come from the LHC, while the masses found by DAMA, CRESST and Cogent, actually seem so low, 10-30 GeV, that the LEP (the previous particle  collider at CERN)  would have already found a neutralino if dark matter was supersymmetric, potentially any dark matter at those energies might be produced in a Higgs decay. One would imagine that a least one Higgs particle not necessarily the light would give mass to both a normal matter and a dark sector of particles, and so decay to both. That mixing might be fast or slow, but now we seem to have early observations of the Higgs, its invisible width (the measurable amount of invisible decays of the Higgs particle), is likely to constrain or identify the heaviest dark particles.