3.3 Non-Baryonic Dark Matter
It will not have escaped the attention of astute readers that the above physical arguments make no mention of the well known controversy regarding the possibility of missing mass in the Universe and the type of matter which might dominate the overall mass density. The most likely form of non-baryonic dark matter to dominate the mass density is known as cold dark matter, the primary candidates for which are in the form of Weakly Interacting Massive ParticleS or WIMP - such as the super-symmetric lepton partners of bosons (e.g. Photino, Gravitino or Higgsino), which have predicted masses ~ GeV. If the major constituent of M in the Universe is in the form of cold dark matter then the growth of fluctuations begins before decoupling: the non-baryonic matter decouples from the radiation earlier and because it feels no radiation pressure can get on with the task of collapsing and forming gravitational potential wells during the radiation dominated era. At the onset of decoupling the baryonic gas, which until this time has been locked to the radiation field by electrostatic forces, falls very quickly into the gravitational potential wells created by the non-baryonic matter. In this case, assuming that the amplitude of density fluctuations decreases with scale, the first structures to form have a mass 105 M - similar to the baryon-dominated case and again set by the pressure of the baryons after recombination.
To summarize, the variety of models represented in this short discussion of the growth of structure suggests that galaxies form by the collapse and subsequent merging of sub-galactic size units over a range of formation redshifts (2 zf 6). Uncovering PGs is one of the few ways of providing constraints on the various possibilities.