The 10 largest companies in the S&P 500 now make up over 1/3 of the index with an avg P/E ratio of 50x. This is the highest percentage since the 2001 bubble. These same 10 companies have accounted for ~80% of the Nasdaq's entire rally this year. The markets are increasing being held up by just a handful of companies, notably the tech companies with a unique ability to integrate AI into their models.

h/t Eric Peters