What's heavier a kilogram of steel or a kilogram of feathers?
Both are the same weight, a kilogram. The difference is in their mass. Steel is denser than feathers, so a kilogram of steel takes up less space than a kilogram of feathers. Does that help?
Is there a unit of density that is internationally used?
Yes, there is an internationally recognized unit of density known as kilograms per cubic meter (kg/m³). This unit is part of the International System of Units (SI), which is a standard system of measurement used by scientists and researchers worldwide.

Density is defined as the amount of mass per unit volume of a substance. Therefore, expressing density in units of kilograms per cubic meter means that we are describing the amount of mass (in kilograms) present in one cubic meter of a given substance.

Other commonly used units of density include grams per cubic centimeter (g/cm³) and pounds per cubic inch (lb/in³), but these units are not part of the SI system and may not be used in international scientific publications or research.