Final answer:
Yellow marrow, found in the hollow medullary cavity of long bones, is the part of the bone mostly made of fatty cells that store energy as triglycerides.
Step-by-step explanation:
The part of the bone mostly made of fatty cells is called yellow marrow. Yellow marrow is found in the hollow medullary cavity within the diaphysis (the long central shaft) of long bones. It is composed predominantly of adipose tissue, which includes adipocytes that store energy in the form of triglycerides. Unlike the red marrow, which is involved in blood cell production, yellow marrow's main function is fat storage.