The Missing Satellites Problem

The standard of model of the cosmology Lambda-CDM predicts that our Milky Way galaxy should have many more satellite galaxies than we actually see: Dwarf Galaxy Problem. It could be that some are actually missing or perhaps they are fainter than expected and hard to detect. See the preprints Notes on the Missing Satellites Problem for a review and also Too big to fail? The puzzling darkness of massive Milky Way subhalos.