The motion of celestial bodies provides another way to calculate mass. When astronomers tally masses based on the motion of stars turning on great galactic wheels, they've discovered there is far more mass in galaxies than is accounted for by calculations based on measurements of luminosity. The same holds true for superclusters. The ordinary kind consists of nonluminous things like dead stars, black holes, and brown dwarfs, which are massive planet-like bodies that are not quite big enough to ignite the forces of stellar combustion. You can't see them, but you could conceivably touch them.
Neutrinos are uncharged subatomic particles.
She says they interact so weakly with ordinary matter that it would take a block of lead several light years thick to stop one. They're ghosts of matter, about as close to nothing as you can get without actually arriving. They're not nothing. Just one second after the big bang, the fiery crucible that forged the universe, they escaped the thermal equilibrium and have been coursing the heavens for 15 billion years. There could be other unthought-of particles that make up dark matter, but Ma believes the relative abundance of neutrinos makes them likely suspects.