The dimness of the black holes located at the center of galaxies surprises astrophysicists, but a possible explanation has been found in the behavior of the plasma they consume. In a hot accretion flow, the gas is ionized to form a plasma. The heavy ions carry most of the mass, and thus of the energy, whereas the electrons produce most of the radiation. But, crucially, in a low-density flow the temperatures of the ions and of the electrons may decouple. Consequently, most of the gravitational energy would be viscously converted into thermal energy of the ions and not radiated away by the electrons. Instead, the gravitational energy is carried with the flow across the event horizon of the black hole. Such a flow leads to a low radiation efficiency even in a highly dissipative accretion disk.