Does not dissolve in the large intestine but soaks up water like a sponge and adds bulk?

The substance you're describing is fiber, specifically insoluble fiber. It does not dissolve in the large intestine but absorbs water, which helps to increase stool bulk and promotes regular bowel movements. This process can aid in preventing constipation and contributes to overall digestive health. Foods rich in insoluble fiber include whole grains, nuts, and vegetables.