It is important to note that, while space is the entity doing the expansion, there is no corresponding expansion within galaxies (their stars are not drawing apart, and in fact over time they tend to contract as the stars move inward towards the usual Black Holes at their centers). A major reason why galaxies (and our own Solar System) do not seem to participate in the general expansion, which applies primarily to the large-scale Universe, is the effect of "local" gravity (from the stars within a galaxy, plus some contribution from nearby galaxies in clusters) that roughly counterbalances the enlargement of space.
The density wave theory explains the arms in grand-design spirals.
-The stars revolve around the galaxy independently of the
spiral arms; the arms are regions where the density of gas is greater than at
other places.
-The analogy would be when you throw a rock in water, waves are created that move across the pond surface; they move outward in concentric rings from where the rock struck. If the pond is also rotating, how do the waves move?
-Lindblad found in the 1960s that they are spiral shaped.
-Russians tested the density wave theory by throwing rocks into water filled pie pans that rotated on a turntable. They found that the water wave patterns were indeed spiral.
-Like water in a pond, interstellar gas sustains “ripples” in galaxies called spiral density waves; they never wind up but orbit in a rigidly defined spiral pattern.
-Spiral density waves move more slowly than the gas and the stars in the galaxy.
-In contrast, sound waves travel faster than the particles in the air.
-Density waves passing through the disk of a galaxy cause the faster moving interstellar gas and dust to pile up temporarily. A spiral arm is a temporary compression, like a galactic traffic jam: individual cars may spend 1 hr in the jam, but the traffic jam may last all day long.
-The spiral arms themselves are waves of enhanced density.
-As the interstellar medium sweeps through the spiral density waves, old gas and dust left behind from dead stars is compressed into new clouds, which then form new metal-rich stars.
-We see spiral arms because they contain lots of bright O and B stars that illuminate vast amounts of gas and dust.
-The massive stars highlighting the density wave explode before they finish passing through it. The longer-lived, lower mass stars fill the space between the arms without emitting much light. Stars as dim as the Sun contribute virtually nothing to the brightness of spiral arms.
-Only 5% more stars are found in the spiral arms than between them.
-It takes enormous amounts of energy to compress the interstellar gas and dust. After a billion years or so, density waves would begin to fade away.
-What keeps them going?
-One theory is that the gravitational attraction of a nearby companion galaxy, as it periodically passes close to the spiral, pulls on the gas, stars and dust and generates new density waves.
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So, matter is actually flowing inward the Galaxy, while waves of gravitational energy (the waters) appear to move outward.