Spiral galaxies have two sleeves more often. The explanation to this fact absent. 

                                                           Two sleeves galaxy  

On  Unified Тheory of Nature  (Formation of galaxies and stars see) the jet flows are formed at a motion of ether from the different sides in the zone with the essentially lower density «ρ». The very uniform distribution of the density of ether is necessary round this macrozone for the formation of the great number of streams (the sleeves). That is very little probable on the such macrowide front round the forming galaxy. The largest probability, hence, at the minimum quantity of the converging streams, i.e. at two.

The galaxy in Hound Dogs. It is known that galaxy NGC 5194 has the hard-curled branches with the cross dark bands. Why the galaxy has such rare species? Convincing answers till now is not available.

Galaxy NGC 5194

From Unified Тheory ("Formation of galaxies and stars" see above) follows that at the uniform distribution of the density of ether «ρ» on the periphery of the future macrovortex (that happens not often, see above) the formation of the galaxy – of the  macrovortex (i.e. of the rotating macrozone of ether) advances of the formation of the mesovortexes-stars. Therefore the stars (on which we see the galaxy and its branches) are formed in the already  arisen – the invisible  twisted macrovortex of ether.

On the contrary, stars (mesovortexes) start to be organised earlier, than a macrovortex - a galaxy at a non-uniform  distribution of density  of ether on a periphery of the future macrovortex because of  the dissimilarties of ether ("Formation of galaxies and stars" see above).  Presence of mesovortexes - stars in this not had the time to be wrapped in an abrupt spiral ether macrovortex does not allow stars because of their Inertia (The essence of Inertia, consequence 2 see), to change close to the initial radial direction of travel in the galaxy central region. In such galaxies the visible branches will appear without hard curling. And as the dissimilarity - (1st fundamental property of World see) then such galaxies there will be overwhelming majority.

Besides the jets of ether, directing in the medial field of the spiral galaxy, should have under Unified Theory (Cross motion see) the rotary movement nearby the longitudinal axis also. 

This movement because of dissimilarity of density of ether ("Formation of galaxies and stars" see above) at the front of the ether macrojet (the galaxy sleeve) is divided into separate flows. These crossflows with stars at observation of such galaxy are perceived in the form of more light bands. Accordingly the fields between them have tone more dark.