For 10 points, name this substance that makes up the vast majority of the universe's mass despite not being directly observable.
E.A. Valentijn and P.P. Van der Werf used their discovery of molecular hydrogen in NGC891 near Andromeda to explain this, and along with mircolensing it might be explained by the discovery of 38 white dwarfs in the Milky Way.  A large portion of it is known to be non-baryonic, and it was first postulated by Fritz Zwicky.  Some candidates for this include photinos and axions, which are Weakly Interacting Massive Particles or WIMPS, or Massive Compact Halo Objects or MACHOs.  For 10 points, name this substance that makes up the vast majority of the universe's mass despite not being directly observable.
E.A. Valentijn and P.P. Van der Werf used their discovery of molecular hydrogen in NGC891 near Andromeda to explain this, and along with mircolensing it might be explained by the discovery of 38 white dwarfs in the Milky Way.  
dark matter