See others' answers about the equilibrium required for BB radiation -- maybe also the density of this matter is not high enough to be a normal BB radiator, or "

See others' answers about the equilibrium required for BB radiation -- maybe also the density of this matter is not high enough to be a normal BB radiator, or "thermalized"? Anyway, the density of this matter could be so low that the amount of energy being emitted/received here per unit area (e.g. per square degree of sky) is too low to be detected by equipment. Also note that the radiation is getting redshifted at every distance, so the signal would be further "smeared" and weakened compared to if it were all at a narrow frequency range.