Key points – we are pretty darn certain it is
present in larger quantities than baryons.
Can provide the extra gravity, extra unseen
perturbation, to let galaxies grow from the
small observed temperature fluctuations.
Unknown what it is!  Recall our previous class
discussion (chapter 2).
Terminology in this game describes “hot” and
“cold” dark matter, indicating whether or not
particles are relativistic when they decouple.