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We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesce, they get hot and the pressure keeps the gas apart and makes star formation really hard). We also see the Bullet Cluster, where two galaxies collided/passed through each other. The gas and dust has been slowed down from collisions, but the dark matter has passed right through. We know this from the gravitational lensing. The lensing happens around the mass of the galaxy, but with the bullet cluster, the lens is off to the side where there is no normal matter, because the dark matter kept going when the regular matter slowed down. In other words, we have some really good evidence for dark matter, but there's a few things going on here and there we can't explain. |