I have been unsuccessfully trying to find a definition of the term "collosional dark matter" or the term "collosionless dark matter". I assume these terms are opposite in meaning so that that finding one definition would therefore be sufficient. I searched the Internet for these terms and found quite a few articles, but in none of the abstracts was a definition given. Merriam Webster gives the definition of "collisionless" as of, relating to, or being a plasma in which particles interact through charge rather than collision https://www.merriam-webster.com/dictionary/collisionlessThe Free Dictionary gives the definition of "collisional" as A brief dynamic event consisting of the close approach of two or more particles, such as atoms, resulting in an abrupt change of momentum or exchange of energy. https://www.thefreedictionary.com/collisional If these are correct definitions in the context of "dark matter", then presumably "collosionless dark matter" means dark matter that interacts via EM, but everything I have read so far about dark matter says this is inconsistent with the lack of any observational evidence for such interaction. Here is a quote from the P Salucci and N. Turini paper: Moreover, the analysis of the CMB fluctuations spectrum and a number of cosmological measurements unavoidably point to a scenario in which a Dark Massive Particle is the responsible for the mass discrepancy phenomenon in Galaxies and Clusters of Galaxies ( Planck Collaboration (2016)).