Based on our current understanding of physics and particles, there are a couple of reasons why a spiral galaxy holds its shade.

First, and probably most importantly, the visible matter in any galaxy makes up only about 10% of the matter within the gravitaional influence of the galaxy. The remaining 90% is made up of dark matter and/or dark energy.

Secondly, it appears that the spiral arms of spiral galaxies are influenced by gravity waves. The compression of the waves is what causes the galaxy's arms to hold their shape.

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