"Our Universe may be full of stars, galaxies, and a cosmic web of large-scale structure, all separated by the vastness of empty space between them, but it wasn't born that way. It's a profound realization that our cosmos hasn't existed in its current form forever. Instead, the Universe came to be this way because it expanded and cooled from a hot, dense, uniform, matter-and-radiation-filled state with no galaxies, stars, or even simple atoms present at the outset."
"Everything, as it exists in its current form, wasn't the way it is today back some 13.8 billion years ago: at the start of the hot Big Bang. Moreover, all that we know today about our place in cosmic history was figured out only during the past 100-ish years (beginning in 1923). However, even with all of this, there are a whole slew of facts most people - even many scientists - don't quite get right about the Big Bang."
The Universe began roughly 13.8 billion years ago in a hot, dense, uniform state and has since expanded and cooled, producing the stars, galaxies, and large-scale structure observed today. Early epochs lacked simple atoms, stars, and galaxies and were dominated by matter and radiation. Observational evidence accumulated over the past century supports that expansion and the hot origin, including galaxy redshifts, relic radiation, and primordial element abundances. Key historical developments include theoretical predictions from general relativity and early proposals of cosmic expansion, followed by observational confirmations that favored the hot Big Bang over alternatives.
Read at Big Think
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