Question: Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see, even taking into account our best bets about invisible stuff like brown dwarfs, "Jupiters", and so on? Is there some missing "dark matter"?

James M Monk answered on 14 Jun 2011:

I was quite skeptical about dark matter a few years ago – it seems like quite a fudge, doesn’t it? Now there is some quite good evidence for it, however. In particular the Bullet Cluster of galaxies is very hard to explain without some sort of dark matter. Two galaxies in the Bullet Cluster are in the process of colliding. The normal visible matter in the two galaxies interacts as you would expect and sticks together.

Thanks to the effect that gravity has on light, it is also possible to image the gravitational field of the two colliding galaxies, and while the visible matter has stopped in the collision, the gravitational fields of the two galaxies carried on moving as if there was no collision. The interpretation is that there is dark matter that does not interact – the dark matter part of the galaxies simply passed through each other.

What this dark matter is, nobody knows, but there is good evidence that it is there. It can’t be ordinary matter like brown dwarves because that would interact in the collision.