From the immediate possibility of Britain leaving the European Union to the longer-term consequences of aging populations, the world's major central banks this week just are not sure what to do next.

Officials from the U.S., Japan, the U.K. and Switzerland all opted to keep monetary policy unchanged this week as they await next Thursday's "Brexit" vote and try to make better sense of the deep-seated forces shaping their economies.

"We are quite uncertain about where rates are heading in the longer term," Federal Reserve Chair Janet Yellen told reporters Wednesday, noting that an aging society and lagging productivity growth suggested borrowing costs should be below historical normal levels.