How much photon is still in flight?
Peebles and Fukugita's excellent "The cosmic energy inventory" estimates that about $10^{-4.3}$ of the total mass-energy is in the form of cosmic background radiation, and $10^{-5.7}$ in the form of radiation from stars.

The estimate that the perturbation due to plasma and other interactions of the background radiation is around $10^{-8.5}$, so clearly most photons have never been captured by anything and are still in flight.