in 1963, Arno Panzias & Robert Wilson detected the afterglow of the Big Bang, known as cosmic microwave background (CMB), which revealed that the Universe was once a very hot, hostile place & together with the expanding Universe concept, led scientists to deduce that the Universe began as an infinitely compact fireball.
the Big Bang describes how this fireball grew to form stars & planets, although it is erroneously thought of as being an explosion in one place, while it is is now thought as not forming from one central ignition point, but rather, that space was first created & then stretched.
according to physics, particles of matter & anti-matter should have been produced in equal quantities & if they meet each other, they would annihilate each other resulting in light - strangely we seem to have much more matter than anti-matter - this still has not been answered.
initially, space was jam-packed with particles which prevented light from escaping, but after 300,000yrs, the Universe cooled to the temperature of the Sun's surface allowing the particles to join to form atoms which do not impede light & thus gradually light started to pour through the Universe
the afterglow from this event formed a bright halo, whose light then began to filter down through space & time.
as we look into space we also look back into time, if it would be possible to see this far, the cosmic halo would be the furthest visible point. Everything beyond this, from the birth of the Universe until this epoch will be invisible to us forever.