Phantom fat is hiding in your cereal bowl. It's the bad boy in your bag of microwave popcorn. It lurks in those low-fat cookies and even in that energy bar. The stuff is called trans fat, created when ordinary vegetable oil is processed into partially hydrogenated oil. It's why Crisco stays solid at room temperature and what makes cakes moist, cookies fresh and crackers crisp. Partially hydrogenated oil is in about 40 percent of the food at the grocery store, including some products most consumers regard as healthy.