Ugh. Unfortunately I can't find it anywhere (there's a couple of things like this lately, that I remember being discussed a lot on HN and being covered in the news, and now have disappeared in the ocean of the internet).

However, these discuss or demonstrate very similar things (especially the first two):

https://www.scientificamerican.com/article/is-inequality-ine...

https://medium.com/swlh/extreme-wealth-inequality-is-inevita...

https://www.osti.gov/biblio/1474734

It's weird to me I can't find it because it was picked up in some large news and economics outlets. Basically, there was an agent-based simulation with really simple conditions that showed that even when everyone starts out with the same income, and you have some really simple exchanges of income that are essentially random, you end up with income distributions that look really similar to actual income distributions in the US.

I think the Scientific American piece discusses some of the same phenomena, and a similar simulation is discussed in the Medium piece. The paper that was going around was very simple in its assumptions, though, and very clearly written.