When analyzing the Solar System, one can see that the planets are aligned in a way that seems as if they are spread out flat like a pancake. But, the solar system isn’t perfectly flat. Instead all the planets (besides Mercury) orbit within two degrees of the same plane. Basically, the solar system is almost flat. Interestingly, our solar system isn’t the only one which has this formation: other exoplanets (planets beyond our own solar system), many galaxies, Black Hole Accretion Disks (the thing around the Black Hole), Saturn’s rings, and other celestial entities seem flat. Even with all this three dimensional space, why does the universe prefer to be flat? Because of particles. The fact that we live in a three dimensional world means that there will always be movement of particles, which is followed by collision. As the particles are moving around, up and down, all particles that bump into each other cancel the opposing movements out, stabilizing and gathering them all onto one surface. While this is happening, the center of the particles continues to spin, due to the Conservation of Angular Momentum (movement of rotation, basically). Over time, this creates the flattened, 2 dimensional disk shape through collisions and crashes, resulting in our flat Solar System!

Source: https://www.youtube.com/watch?v=tmNXKqeUtJM