Since Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion.
The scientific community was once divided between supporters of two different theories, the Big Bang and the Steady State theory, but a wide range of empirical evidence has strongly favored the Big Bang which is now universally accepted.
On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).
If the known laws of physics are extrapolated to the highest density regime, the result is a singularity which is typically associated with the Big Bang.
Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
The Lambda-CDM model is the current "standard model" of Big Bang cosmology, consensus is that it is the simplest model that can account for the various measurements and observations relevant to cosmology.
Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together.
The continuous expansion of the universe implies that the universe was denser and hotter in the past.
After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms.
Giant clouds of these primordial elements later coalesced through gravity in halos of dark matter, eventually forming the stars and galaxies visible today.