This beautiful image, ablaze with the light from over 100 billions suns, perfectly captures the thin aspect of large galactic discs.  Most spiral galaxies are incredibly slender relative to their diameter. The thickness of our own Milky Way is only 1% of its total 100 thousand light year diameter.  So next time you see the band of the Milky Way overhead you can roughly approximate its width to be 1000 light years.

The dynamics of gravity and centrifugal force can explain much about the formation of spiral galaxies but the fine detail and the rotational speed of the various bands of stellar material are still shrouded in some mystery.  Without concepts like ‘dark matter’, for instance, the outer portions of many galaxies are rotating so fast they would simply rip themselves apart.

Many people are uncomfortable with terms like ‘dark matter’ and ‘dark energy’ because they’re ‘inferred’ phenomena rather than being directly detected.  This might be so but science is riddle with such pre-emptive constructions.

Take air for example.  For untold millennia people knew about the existence of this invisible substance which would fill people’s lungs and impede the flow of water ‘an air lock’.  However, it would have to wait until the discovery of modern chemistry for air to be properly defined as a mixture of several atomic elements (mainly oxygen and nitrogen).