Interstellar space is predominantly a dark environment due to the vast distances between celestial objects. There is no atmosphere or scattering agents like on Earth to spread light, and the primary sources of illumination are distant stars and occasional nebular emissions. Therefore, the perceived brightness in interstellar space is extremely low.
To quantify the energy of photons in one cubic meter of interstellar space, we must consider the Cosmic Microwave Background (CMB) radiation, starlight, and other forms of electromagnetic radiation present. The CMB predominantly fills interstellar space, peaking in the microwave region at roughly 160.2 GHz. Its temperature is approximately 2.725 Kelvin, which translates to a small energy density.
The energy density of the CMB, which accounts for most of the photon energy in space, is about 4.19 x 10^-14 J/m^3. This is derived from the Planck distribution as applied to the temperature of the CMB. When including contributions from starlight and other galactic and extragalactic sources, the total energy density might slightly increase, but will still remain quite low. These sources contribute primarily in the visible, infrared, and ultraviolet spectra, but their combined energy density is negligible compared to that of the CMB. Overall, the average combined energy of photons in 1 m^3 of interstellar space remains on the order of 10^-14 joules.