Non-Baryonic Dark Matter
I’m no expert on this stuff (and in some
sense NO ONE is).  Particle physicists
play in this area more than astronomers.
Leading candidates include
Axions.  Cold, low mass, avoid strong CP
violation.
Neutrinos.  Hot, low mass (getting better
constrained), lots of them.  SN helps.
WIMPs.  Gravitino, photino, etc.
Mirror Matter.  May use in my next novel.