They would have to be very non reacting with ordinary matter, because we don't see weird reactions going on all the time. Neutrinos Neutrinos are non-baryonic particles that are known to exist. At all times three hundred of these particles, left over from the big bang, course through every cubic centimeter of space. There are more neutrinos coming from the sun as a byproduct of nuclear fusion. Recently neutrinos were measured to have mass, which means that they definitely make up at least part of the dark matter halo.
Neutrinos have no charge.
They have a very light mass. The exact value of the neutrino's mass is uncertain, which is why the uncertainty of HDM's contribution to the density of the universe is so large. We know a lower limit of the mass because recent experiments have determined that neutrinos have a mass of at least .054 eV. The upper limit comes from structure formation. Structure formation shows that in order for the universe to be as clumpy and have as many large scale systems as it does there can only be a limited amount of hot dark matter in the universe. If there were more HDM the universe would be much more smooth and we would not see galaxies or galaxy clusters.