Thanks for the idea pj!
https://en.wikipedia.org/wiki/Big_Bang
The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution.[1][2][3] The model describes how the universe expanded from an initial state of very high density and high temperature,[4] and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, large-scale structure, and Hubble's law – the farther away galaxies are, the faster they are moving away from Earth. If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity. Current knowledge is insufficient to determine if anything existed prior to the singularity.
Timeline of the metric expansion of space, where space, including hypothetical non-observable portions of the universe, is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).
Part of the detailed descriptions of how things proceeded and how they figured them out are incredible. The people that figured those things out are way smarter than me.
However, there are different versions of this theory and still some unresolved issues............so uncertainty
There has also been vigorous debating and discussions on this theory for many decades, which is a good thing for science, though the general theory seems to be on rock solid proven ground.
"[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features."
The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by BBN. More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures,[75] These are sometimes called the "four pillars" of the Big Bang theory.[76]
Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations.[77] Remaining issues include the cuspy halo problem[78] and the dwarf galaxy problem[79] of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.[80] Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.
This site offers an easier to understand explanation: