Equities pushed to new highs even as the economic backdrop stayed stubbornly uncertain.

The rally has puzzled even seasoned strategists, who see plenty of reasons for caution.

A handful of giant companies have done much of the heavy lifting.

Climbing a wall of worry

Beneath the record, the market is narrower than it looks.

"The index is at a record. Most stocks are not."

Analysts point to a handful of forces driving the story forward:

  • A narrow, top-heavy rally
  • Optimism on rate cuts
  • Nagging economic doubts

Whether the breadth improves may decide how long the run lasts.