Data: FactSet; Chart: Matt Phillips/Axios The momentum trade is having a terrible start to the second half. Why it matters: Momentum shares drove the market gains early in the year, and it will be harder for the rally to continue without their participation. How it works: Momentum is one of the "factors" some mathematically minded investors, known as quants, use to describe and organize the market according to certain well-understood market trends. Momentum stocks are defined, more or less, a