# Dark Matter Question

 P: 52 I think there is more to AdkinsJr question than what Janus responded to. In principle Dark Matter (DM) could be in the form of planets or dim stars (e.g. brown dwarfs) or black holes. Because the average distance between these objects would be so large,you won't always see starlight dim (unless a DM object eclipses a star). My guess is that people have ruled out brown dwarfs and planets as being DM for two possible reasons. First off, planetary and stellar formation theories don't allow for most of the matter in a galaxy to collapse into brown dwarfs and planets. Second, someone may have done a direct search for brown dwarves that showed there aren't enough to explain dark matter. A much more respectable possibility is that massive compact objects (e.g. neutron star or black hole) comprise DM. Astronomers can search for these compact objects through microlensing effects. When a compact object passes in front of a star the compact object's gravity can act like a lens and magnify the stars light for us. So by keeping our eye (i.e. telescope) on a bunch of stars we can look for spikes in their brightness which could be these compact objects. This type of survey has been carried out and has ruled out DM being black holes with a mass of $10^{-8}M_{sun}$ (according to the wikipedia entry on this subject). Thus, most astronomers think that DM is comprised of particles that lie outside our current understanding of particle physics.