The least accurately-known of all of the physical constants of the universe is the Newtonian constant of gravitation, G, often called ‘Big G’. This constant determines how strong the gravity of one body is on another, and vice versa. However, the best determinations of this constant still have an uncertainty of 0.15%.
The physical constants of the universe are fundamental numbers, such as the charge on an electron and the speed of light in a vacuum.