gravity - Why aren't there spherical galaxies?

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies:

• Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out


• Ellipticals - look like a big disk of stars and other matter


• Lenticulars - those that are somewhere in between the above two


• Irregulars - galaxies that lack any sort of defined shape or form; pretty much everything else

Now, from what I can tell, these all appear to be 2D, that is, each galaxy's shape appears to be confined within some sort of invisible plane. But why couldn't a galaxy take a more 3D form?

So why aren't there spherical galaxies (ie: the stars and other objects are distributed within a 3D sphere, more or less even across all axes)? Or if there are, why aren't they more common?

Answer

All matter in the galaxy has to rotate (not necessarily in the same direction) so that a centrifugal force acts. Without the centrifugal force, all matter contained in the galaxy will collapse into the center of the galaxy due to gravitation. The rotation happens about an axis, a line about which all matter revolves in the galaxy. Now, the manner in which all the matter revolves around that axis is planar. Why is it planar and why does it have to rotate about an axis only? The answer to this question will decisively clear that doubt.

But how does the planar galaxy continue to retain planarity for billions of years?

Let's imagine that a planar galaxy has a few bodies which don't revolve around the central axis and have their own axis of rotation. In any direction perpendicular to that axis, centrifugal force keeps the body from collapsing into the center of the galaxy. In any direction parallel to that axis however, there is no such centrifugal force; but there is a component of the gravitational force from the matter contained in the planar galaxy below. This component of gravitational force keeps pulling the body toward the plane, and there is no force to stop it. Thus, even this body will eventually join the galactic plane. All such fringe bodies which do not obey to the galactic plane will be attracted by gravity to eventually join the plane. Therefore the galaxy manages to maintain planarity.

As Rob Jeffries pointed out, there are galaxies that are of spherical and other three-dimensional shapes. There, however, since there is no pre-existing plane of rotation, nothing is causing the matter to collapse into a plane. Therefore, those galaxies retain their three-dimensional shape.