The early universe was super dense and light couldn't escape it. However, at some point it got cool and sparse enough that neutrinos could.

The CNB (Cosmic Neutrino Background), not to be confused with the CMB (Cosmic Microwave Background), composed of photons and which you mention in another reply further down this thread.

As neutrinos were able to liberate themselves in a straight line from the post-Big Bang pinball soup earlier than photons, the CNB would let us peek further back in time.

The holy grail would be the Cosmic Gravitational Background, basically the fading "GONNNNNG...!" of spacetime ripples created at the Big Bang itself, as if it was a bell that was struck.

This summer the detection of low-frequency gravitational waves was announced, and while that's about "a" gravitational background, it's not "THE" Gravitational Background, by which I mean not the one from the Big Bang itself.