Dark Matter only: detected through its gravitational attraction, has mass, most common form of mass in the halo of the Milky Way Galaxy, likely to consist of subatomic particles Dark Energy only: inferred to exist from the measured acceleration of the universe's expansion, the proposed source of a universal repulsive force BOTH: the universe contains more of this than it contains matter made from atoms, we do not know what it is made of NEITHER: blocks light from stars behind it, "dark" because it emits only infrared light