Heat is atoms vibrating and bumping into each other. The way heat typically transfers and dissipates on earth is atoms bump into other atoms, transferring the energy to those. But this requires things to be in physical contact. If there is nothing to touch, heat can’t be dissipated the normal way. This is how thermoses work; there is a vacuum in between layers of metal, heat can’t cross the vacuum which makes it harder for heat to escape.

The glow you see from something “glowing hot” is black body radiation . Basically everything emits light at a wavelength dependent on how hot it is. Things that aren’t yet “glowing hot” are just emitting light that is at a lower wavelength than we can see, like infrared. That’s how some night vision googles work: they can detect the difference in infrared light emitted by something at body temperature and something at room temperature.

Black body radiation carries energy, cooling the thing that emitted it and warming whatever ends up absorbing it. In space, this is the only way to dissipate heat. Satellites often have large heat radiators designed to dissipate heat in this way as efficiently as possible, but it’s still much, much harder than on earth, because you basically have a hot thing you want to cool down but you’ve put it in a vacuum thermos.