It's not just galaxy rotation. That's how people first noticed it. Since then there's stuff like this: http://en.wikipedia.org/wiki/Bullet_Cluster http://en.wik

It's not just galaxy rotation. That's how people first noticed it. Since then there's stuff like this: http://en.wikipedia.org/wiki/Bullet_Cluster http://en.wikipedia.org/wiki/Bullet_Cluster Also there's apparently information about the universe's composition to be gleaned from the CMB anisotropies; its all somewhat beyond me, but that curve certainty seems to fit extremely well, I'll give them that. http://www.astro.ucla.edu/~wright/CMB-DT.html http://www.astro.ucla.edu/~wright/CMB-DT.html The combination of all this stuff is apparently pretty convincing. EDIT: I see that your comment is actually referring to the bullet cluster more than the rotation curves. It's all pretty tenuous in the grand scheme of things, I agree, but then so much of cosmology is. I guess they have the benefit of the string people next door making them look reasonable by comparison.