Several arguments are presented indicating that the apparently non-luminous matter forming massive halos of spiral galaxies is not baryonic. There are difficulties with a halo dominated by gas, snowballs, dust and rocks, jupiters, low mass stars, dead stars and neutron stars. Also, halos may not be composed of black holes unless they are either extremely efficiently accreting or primordial. Consequently, it appears that a significant fraction of the universe may be in the form of massive neutrinos, gravitinos, monopoles, etc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25181/1/0000620.pd
