Here's the chart:
Nvidia's share of the S&P 500 index is now bigger than the entire energy sector (3%), the entire utilities sector (2.5%) and the entire materials sector 2.3%, added together.
That means that when you are buying an index tracker, you arn't buying a properly diversified portfolio, instead you are overweight Nvidia.
What happens when Silicon Valley decides they're overpaying for chips, and they can get more bang for their AI buck by reconfiguring their software to process data more efficiently?
China, which hasn't got access to the best chips, has been forced to take that approach by necessity. And DeepSeek shows they're had some success.
Google, Microsoft, Meta and co have likely already set up teams to look at this, with a view to saving costs.
At which point Nvidia's share price will start to descend to earth.
The headlines will be "The stock market is crashing!" when in reality just one stock will be reverting to the mean.