system
Given the following post, respond as if you're commenting on Reddit:
human
How did the first atoms come into existence and how do we know that?
gpt
From about 1915 to 1926, work was done which showed that all the galaxies around us, were moving away from us - and that the speed with which they moved was proportionate to how far away they are.    That observation alone implies the were all at the same starting point, at a finite time in the past.   


This turned out to be consistent with the newly proposed (1915) theory of General Relativity - which , prior to this, had the problem that it couldn't produce an infinite-age, static (non-expanding or collapsing) universe.  

This leads to the conclusion that, as you trace time going backwards, the Universe gets denser, and hotter; inevitably, at some point, the universe is so dense and hot, that the light elements we see around us would break apart, first into their constituent neutrons and protons, and then those break apart into their constituent quarks.  

Now, run the same scene forward in time: as the Universe expands, it becomes less dense, and cooler, and the quarks first join into protons and neutrons, and then these join into the light elements.   In fact, based on the binding energy of these elements, you can predict what the ratio of the density of the lightest elements are to each other -- and it does a \*pretty good job of doing that.   


  
\*Note: "Pretty good job" means different things to physicists than it does to, say, a cook.