Why are the galaxies planar?

I think a better question would be-Why did gravity cause planets to form spheres but larger systems like solar system/galaxies to take a flat planar disc shape? read more

Planets are continuous mass distributions and there is another effect to keep in mind - that is pressure, when gravity pulls you down the center of the planet the upward pressure is the force resisting gravity and keeping you from falling to the center. read more

Why are spiral galaxies planar (I know there are exceptions to the rule, I'm just trying to understand the why). From what I understand, it is a bit of a battle between gravity, centripetal acceleration and conservation of angular momentum. I understand these concepts but I can't figure out why this results in a planar galaxy. read more

Here the force resisting gravity is the centrifugal force ( which is circularly symmetric) and this causes disc shaped or planar systems. The difference in shape between planets/stars and solar systems/galaxies comes because of 2 principles—Equilibrium and Symmetry. read more