Photon Density of the Universe

The current temperature of the observable universe is about 2.7 K. What is the photon density of the universe in $\frac{photons}{{m}^{3}}$ ?

First calculate your answer in the form $a\times { 10 }^{ b }$ , then input the value $\left\lceil a \right\rceil +b.$

Note: we assume that the universe is isotropic.