The states of matter found on Earth include solids, liquids, and gas. Gas is the least dense state of matter as the particles are far apart. After gas, liquid is the least dense. The particles are closer but far enough apart to allow flow. Finally, solids are the densest.
Their particles are packed so tightly together they can barely move making for a solid material. The density of a solid is tangible and is usually associated with a chemical property. A liquid or a gas can be easily compressed to fit into a specific area whereas solids can't be manipulated as easily.
Solids have the highest density because they are tightly packed. The atoms and molecules in a solid have fixed positions. They don't move around like atoms or molecules in a gas or liquid do. Density is a tangible property that is often correlated with chemical composition.
The density of matter is communicated in the form of mass per unit of volume. Solids have a definite shape and volume can't flow. Solids cannot be easily compressed. There are also a lot of particles in a small volume, and they are held tightly together by strong forces of attraction.