ArcticWorld explains that density is a measure of how tightly packed a particular substance is. Objects that are denser contain less empty space in them.
ArcticWorlds goes on to explain that in order to measure the density of a substance, the mass, or weight of the substance must be determined and divided by its volume, or the amount of space the substance takes up. No matter where a substance is in the universe, its density remains unchanged. Water is used as a standard for density. All objects less dense than water float, while those objects denser than water sink. Ice cubes in water show the differences in density. The cubes float because they are less dense than the water, meaning they possess more open space, or air.
Brian Kross of Jefferson Lab explains that a one pound plastic sponge, if melted down completely, will be smaller but still weight one pound. The sponge decreases in volume but becomes denser as the air pockets are removed.
Cole Miller of the University of Chicago points out that the most dense substance known is the matter found within a neutron star. In fact, a lump of neutron star matter the same size of a cube of sugar would weigh as much as all of humanity.