The substance with the highest density at room temperature is osmium, a hard, blue-gray transition metal.

Here's why:

* Density is defined as mass per unit volume. Osmium has an exceptionally high atomic mass and a very compact atomic structure, resulting in a very high density.

* Room temperature is generally considered to be around 20-25°C (68-77°F).

While other metals like iridium and platinum also have very high densities, osmium consistently takes the top spot.