Caption: A schematic diagram illustrating the evolution of observable universe according to the Λ-CDM model plus an early phase of cosmic inflation.

The 3-dimensional size scale of the observable universe is represented by horizontal 2-dimensional slices and cosmic time is represented by the vertical direction.

The cosmic inflation rapidly expanded the early observable universe from early minute size.

After cosmic inflation and at 13.75(11) Gyr ago (see Wikipedia: Concordance model: Parameters), the Big Bang happened---as I use the term---in which nucleosynthesis cooked up the primordial abundances of hydrogen, deuterium (heavy hydrogen), helium, lithium.

A decelerating phase of universal expansion followed cosmic inflation which cooled and rarefied the observable universe.

About 400,000 years after the Big Bang recombination epoch happened---the hydrogen became unionized neutral hydrogen gas.

The neutral hydrogen gas was largely transparent causing the universal electromagnetic radiation (EMR) decoupled from matter and flow freely in space cooling to become the cosmic microwave background (CMB) of the present observable universe.

At some point between the Big Bang and the present observable universe, the universal expansion started to accelerate giving us the present accelerating universe.

The Λ-CDM model accounts for the observable universe and its known evolution through time very well. It is probably NOT the last word, but many features of it may be right.

Cosmic inflation (which I do not consider part of the Λ-CDM model) is a more speculative theory and may turn out to be just plain wrong---but it's stuck around since circa 1980---and so it has some staying power.

Image linked to Wikimedia Commons.
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