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i want to know why galaxies are spiral in nature.. let us say there is some sort of intense mass (black hole?) at the centre of our milky way galaxy. the intense gravitaional pull is keeping evey solar system to revolve around it. thats clear, because that is what is happening in our solar systm too. but the planet goes round around the sun (ellipticaly, to be correct). but i recently saw pics of galaxies and they were spiral..? if the gravitational pull is keeping every solar system to revlove around the centre of galaxy, what different force is giving them the spiral shape? i mean, spirals are created in viscous material (water, air) but the universe is all vacuum..? then how are galaxies spiral in nature?