"Black body radiation" is the physical process by which you "dissipate" (the correct word here is "radiate") heat in space.

In space you can't just have the heat be passed from the radiator to some "substance" that fills space (like on Earth the heat is passed to air or to water that then gets released to the environment) because almost all of space is empty of matter (not exactly: there's incredibly low density stuff in it, mainly ions, but such low density means pretty much no available mass to sink the heat), so the only way for that heat to leave is the natural physical process of a warm body emitting photons merely because of its temperature (the wavelength of which depends on temperature) which is called Black Body Radiation.

As others have pointed out, it's a way less efficient process that dissipating heat by it being passed from the radiator directly to some substance that's part of the environment (i.e. transmission).