I presume the volume of the Universe. We can measure average density of residual radiation but getting the total value of energy requires multiplying by volume.

I presume the volume of the Universe. We can measure average density of residual radiation but getting the total value of energy requires multiplying by volume. Much of the universe's volume may be hidden behind a cosmological horizon (basically, the idea that as the universe's rate of expansion accelerates the greater the distance from us (Hubble's constant), eventually the rate of expansion exceeds the c and thus light from that part of the universe can never reach us, making it unobservable).