Open universe. Artwork showing the geometrical curvature of an 'open' universe. The galaxies are shown on a curved grid, which represents the overall geometry of the universe. This type of universe is also called a 'saddle' universe, after the shape of the curvature. The curvature of a universe is determined by the amount and density of matter and energy. In an open universe, the amount of matter is insufficient to prevent the universe expanding forever. The curvature is said to be negative and the volume is infinite.