DM is as yet an unknown form of matter that is needed to explain anomalous  rotational curves of galaxies, as well as structual galactic formation in general. Without it, means that our whole picture and theories about gravity is wrong. That is viewed as unlikely as it works so well in all other aspects...hence the unknown missing non baryonic mass hypothesis. In actual fact the name DM is more an expression of cosmological ignorance then anything else, much as the old ancient cartographers