You might think we should be able to see light going back to the first moment after the Big Bang. But, tracing back the state of the universe, we believe there is a moment when no light could travel through space because space was opaque. Photons just found themselves buffeted between one particle and the next. It took 378,000 years following the Big Bang before the density of particles dropped sufficiently for the first photons to start their uninterrupted journey through space.
