We know that the speed with which the planets go around our sun decreases as a function of the distance they are from the sun. Since stars are not planets, and presumably do not behave like planets, why do we relate the revolving of planets around our star, on the one hand, with the revolving of stars in a spiral galaxy, on the other hand? When we find that stars move with constant speed regardless of distance from the centre of the galaxy, why does this surprise us on the basis that planets move with differing speeds around the centre of a solar system? In other words, are we not comparing apples with oranges when we have to posit dark matter as an explanation of stars all moving at constant speed unrelated to distance? I suppose that an object revolving around something else is common to both situations, but a planet is not a star and our sun is not the centre of a spiral galaxy. So, why reach for an explanation of the latter on the basis of our knowledge of the former?
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