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You can define the direction of 'spin' by other properties - usually the galaxy's magnetic poles, so you can define north and south and hence 'up' and 'down' and the 'spin' in relation to those.
Someone mentioned something about that everything in the Universe is 50:50...
Back to Quantum Mechanics and Relativity and cosmology....
One of the consequences of a current interpretation of QM and R is that after the Big Bang in which basically everything was energy and then as the Universe cooled and expanded, particles were formed from the energy. Included in this were electrons-positron pairs, most of which annialated each other to form gamma photons. However there was a slight excess (about 0.0000...001%) of negative electrons and so as a result atoms were able to form from the protons and negative electrons.
In other words, for some reason, the Universe had 'handiness' ie a preference for one way over the other.
Similarly, many molecules have a handiness which prefers their arangement for one isomer over another, and this is the root cause of why many sea shells, vines, or whatever prefer one spiral form over the other. I think from memory... clockwise...? is prefered?
Anyway, the other thing is that there possibly is no actual spiral in galaxies, rather it is an effect of a shockwave that propogates through that causes density variations in interstellar matter. This in turn has two effects - one is it gives the appearance of a spiral, the other is that it actually generates a spiral as the density variation leads to new nebular and star formation as the shockwave moves through space. Rather than the stars moving around in a spiral, the shockwave generates the illusion of a spiral as new stars etc are formed. I suppose an analogy is the formation of a lenticular cloud forming over a hill, although this is the opposite case in which the cloud stays stationary due to a stationary disturbance despite the air moving.
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