Unsold stuff is piling up.

The unsold cars and trucks piling up at dealerships and assembly lines as consumers cut back and auto companies scramble for federal aid are just one sign of a major problem hurting the economy and only likely to get worse.

The only way to stop the downward spiral is to boost demand. That’s the point of the stimulus package.

The world is suddenly awash in almost everything: flat-panel televisions, bulldozers, Barbie dolls, strip malls, Burberry stores. Japan yesterday said its economy shrank at an 12.7 percent annual pace in the last three months of 2008 as global demand evaporated for Japanese cars and electronics. Business everywhere are scrambling to bring supply in line with demand.

Downsizing can be tricky, though. No one knows how much worse the economy will get, and while everyone waits for the recession to peter out, businesses are grappling with how to cut costs and survive without sabotaging their ability to grow when the economy picks up.

And there is a lot to cut.

“There is over-capacity in everything,” from “retail to manufacturing to housing,” said Richard Yamarone, chief economist at Argus Research. “If capacity is too large, you don’t need that many people employed, which is another reason we’re seeing such high job losses.”

As long as capacity far outstrips demand, businesses have little reason to expand, buy new equipment or hire workers. Even if the government funds bridge repairs and banks step up lending, many industries still have to go through massive restructuring before growth can resume. But executives say they have to tread carefully. If they put off critical investments in technology or research for too long, they could hobble their recovery and even the economy’s.