The AI boom in the U.S. stock market technology sector has boosted the $NASDAQ(.IXIC)$ and $S&P 500(.SPX)$ this year.To put things into perspective: 10 stocks have contributed 90% of $S&P 500(.SPX)$ 's 13.0% gain this year, the most concentrated market ever, Bernstein says. The stocks are $Apple(AAPL)$ (up 41.45%), $Microsoft(MSFT)$ (+38.37%), $NVIDIA Corp(NVDA)$ (+170.17%), $Alphabet(GOOG)$ (+40.14%), $Amazon.com(AMZN)$ (+50.68%),