10. Energy density in a blackbody radiation field is u=aT⁴, where a=4sigma/c. For T=2.7 K, this gives u=4.0x10^-14 J m^-3.

The energy density of sunlight at the Earth is u=f/c, where f=L/(4 pi d²) is the flux of sunlight. This gives u=4.6x10^-6 J m^-3; i.e. 8 orders of magnitude larger.