The main criterion for something to exist is that all the conservation laws, such as energy, momentum, and certain other quantum mechanical parameters, are obeyed.
As far as we can tell, and many experiments have shown, photons have zero rest mass. They are, however, real existent objects that you can see and measure. Although they have zero rest mass, they carry energy and momentum. You can, sort of, slow photons down by sending them through a refractive medium but you really can't stop them. They do interact with ordinary matter and stop giving up their energy and momentum in the process.
A photon, once it exists, doesn't care a whit about the temperature of its environment.
(published on 12/16/2010)