When cream is separated from milk—

Density is mass per unit volume, and the density of a mixture depends on the densities and relative proportions of its parts. Milk is an emulsion in which fat globules are suspended in an aqueous phase of water, proteins, and lactose; fat is markedly less dense than this aqueous phase.

Cream is the fat-rich layer skimmed off milk. Because fat is the lighter (less dense) component, removing it leaves behind a mixture with a proportionally larger share of the denser water-protein-lactose phase, so the remaining (skimmed) milk becomes denser than the original whole milk.

This matches measured values: whole milk is close to 1.030 g/mL and skimmed milk rises to about 1.033–1.035 g/mL, while cream — being enriched in fat relative to the milk it was separated from — is consistently less dense than both. It is also why dairies use a lactometer, which reads a higher value for skimmed milk than for whole milk, to check for fat content and adulteration.

• A decrease in density would require removing a component denser than what remains — fat is the less dense component here, so removing it cannot pull the density down.

• Density stays the same only if the removed portion has the same density as the rest; since fat's density differs sharply from the aqueous phase, taking it out necessarily shifts the mixture's overall density.

• Viscosity and density are different properties: fat globules are what thicken milk's texture, so removing them (as cream) actually lowers viscosity, making skimmed milk thinner — not more viscous.