The S & P 500 closed above 6,000 for the first time on Monday, after touching 5,000 on Feb. 9. Since then, energy firm Vistra, which was included in the S & P 500 in May, and data analytics firm Palantir, which was included in September, have led the rally with gains of 237% and 147%, respectively. Both companies have been boosted by the artificial intelligence boom.