13.80 – 13.05 Bya (billion years ago)
Knowing what happened at the very beginning of the Universe is speculative. It depends on what the theory of quantum gravity looks like, which is up in the air. The theory of inflation (insanely fast growth before 10-32 seconds , after which the universe settled down to merely explosive growth with the Big Bang) may explain why the universe is flat, uniform, and not very lumpy. In 2014, it looked like we had direct evidence for gravity waves generated by inflation, going back just 10 sec from the beginning of the universe. But the jury is still out on this.
Later developments are more generally agreed on, although some of the exact times may need revision in the future. Strikingly, a lot of familiar astronomical objects, including stars and galaxies, are already around within 100’s of million of years. However early stars are short on metals (to astronomers, anything heavier than helium counts as a metal), and the early Milky Way is dispersed and fuzzy, not the barred spiral galaxy we know today.