The current Lambda CDM model may explain a great deal about the evolution and the chronology of the events that occurred in our Universe but it doesn’t paint the complete picture.
We know of the cosmic inflation that happened followed by the Big Bang itself however how these two are coherently connected has so far defied all our attempts to explain.
During the inflationary period, within less than a trillionth of a second, our universe grew from an infinitesimal point to an octillion (that’s 1 followed by 27 zeroes) times in size, which was followed by a more conventional and gradual period of expansion, nevertheless violent by our standards, which we know as the Big Bang.