If you take a look at a spiral galaxy, it immediately suggests a vortex ring. The understanding of why there are spiral arms can be straight foward when seen as to be a result of its formation. First we might have some a bit featureless cluster of stars or durst. This creates a strong gravity which begins to attract other 'particles'. The 'particles' in question can be seen as to be those that formed an initially featureless cloud which was also slowly spinning initialy. The gravity then lead to the compression of this 'cloud'. As it got compressed, it begun to spin faster and faster. So the explanation exactly follows the steps in the explanation for hurricane etc. So the similarity between a galaxy and some hurricane will not be a coincidence.

This simple explanation is, however, rejected in mainstream science because it is thought that galaxies must be billions of years old! This means that the appearance of a galaxy must never be in such a way as to look like it is still under its early development. Galaxies, so it is thought, must have formed eons of years ago. So if it spiralled during the process, the whatever spiral structure formed immediately underwent endless windings until the galaxy grew featureless as stars settled into stable orbits rather than spirals. So pay attention that astronomers did not begun by any observation. They begun by supposing that galaxies are billions and bilions of years old. After which they must explain observations to fit into this idea of age of galaxies! Any explanation that 'confirms' that galaxies are zillions of years old will then be welcom regardless how 'Ptolemaic' it is!

So the explanation that they offer for the spiral arms goes this way: The stars or dursts or clouds or whatever are initially moving in ellipses. The axis of the bigger ellipses are tilted slightly from the axis of the smaller ones. So the larger the (elliptical) orbit, the more tilted the ellipse. This arrangement results in the orbits comming closer to each other at some areas and moving away from each other at the other areas. The regions where the orbits comes close together will be the denser regions. This fits together nicely to form what looks like two spiral arms!

There are at least two reasons we must reject this concoction of theirs. One is that it requires a miracle to allign orbits in such a manner. Why should we have orbits that are so arranged like marching soldiers, with stars aligned so well along the orbits? Why would myriads of stars share the same orbit? Who was the 'commander' of such an 'army' of stars marching in such a synchronized manner? The next reason we must reject the concoction is that if stars were actually along ellipses, then such ellipses would be visible! The galaxies would appear exactly like the way they draw them, with elliptical lines, and then we would see how they approach each other to form spiral arms that are superimposed on elliptical rings!

Notice that you can't fail to notice that Saturn rings are indeed rings, rather than looking like some 'clouds' around the planet 

Similarly, we cannot fail to notice that the commet tail is, indeed made of particles that are streaming out along straight lines.

There is a reason for this ease in noticing when all substances are aligned, orbiting or spiralling. It requires unbelievable conspiracy to hide this fact! You must be carefull to first create endless lines crisscrossing each other, and then place the particles on junctions so that you can't tell what line they belong in. Otherwise a chaotic sprinkling of pariticles so that they align will always show lines that are superimposed on chaos.