When we look at the movement of the planets in our solar system, we see that they all match our equations of orbital mechanics. They all have a balance between gravity and centripetal acceleration, so they follow stable orbits. If we discovered an exact copy of our solar system, but with planets that moved twice as fast in the same orbits, that wouldn't make sense with our equations. They would predict that gravity could not be strong enough to balance the centripetal acceleration, and the planets should fly apart as we watch. If that doesn't happen, it means one of our assumptions is wrong. It could be that the sun is actually much more massive than our sun, or something stranger is going on.
When we look at a galaxy, plug in the positions and velocities of all the stars we see, and try to simulate how it will behave, the simulation typically predicts that the galaxy is going to fly apart immediately. Since that's not happening, we know something is wrong with our explanation. The simplest fix is if there's a lot of mass that we can't see, but if that's not true, then we're in the much more uncomfortable situation of our equations being wrong or incomplete.
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Their behavior doesn't follow our model of gravity. If we add a bunch of extra mass it follows it perfectly, however.
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When we look at the movement of the planets in our solar system, we see that they all match our equations of orbital mechanics. They all have a balance between gravity and centripetal acceleration, so they follow stable orbits. If we discovered an exact copy of our solar system, but with planets that moved twice as fast in the same orbits, that wouldn't make sense with our equations. They would predict that gravity could not be strong enough to balance the centripetal acceleration, and the planets should fly apart as we watch. If that doesn't happen, it means one of our assumptions is wrong. It could be that the sun is actually much more massive than our sun, or something stranger is going on.
When we look at a galaxy, plug in the positions and velocities of all the stars we see, and try to simulate how it will behave, the simulation typically predicts that the galaxy is going to fly apart immediately. Since that's not happening, we know something is wrong with our explanation. The simplest fix is if there's a lot of mass that we can't see, but if that's not true, then we're in the much more uncomfortable situation of our equations being wrong or incomplete.