I know heat as the average kinetic energy of the particles in a system, is this correct? And that heat is transferred by transferring energy, which causes particles to go slower/faster, is this correct? But what exactly is heat? Is heat made up of smaller particles, such as light is photons? I still don't really understand what exactly heat is. Also, in vacuum, heat is through neutrinos, they carry the energy, is this right? Do photons in vacuum also transfer heat?