On the separation between baryonic and dark matter: evidence for phantom
  dark matter?
  The recent years have seen combined measurements of X-ray and (weak) lensing
contours for colliding galaxy clusters such as, for instance, the famous
"Bullet" cluster. These observations have revealed offsets in the peaks of the
baryonic and (dominant) gravitational matter component of order ~(100-200) kpc.
Such discrepancies are difficult to explain using modified theories for gravity
other than dark matter. Or are they not? Here we use the concept of "phantom
dark matter" that is based upon a Newtonian interpretation of the MONDian
gravitational potential. We show that this idea is in fact capable of producing
substantial offsets in idealistic density configurations, involving a uniform
external field. However, when analysed in a MONDian cosmological framework we
deduce that the size (and probablity) of the effect is too small to explain the
observed offsets found in the most recent observations, at least in the
simplest incarnation of phantom dark matter as discussed here. The lensing
centers in merging galaxy clusters are likely very close to the centers of true
mass even in a MONDian cosmology. This gives the support to the idea that
neutrino-like non-collisional matter might be responsible for the observed
offsets of lensing and X-ray peaks.
