So we all know "dark matter" exists, and it has been calculated that each galaxy has this dark matter which has 4x the mass of a galaxy, so my question is how did they calculate this "dark matter" having 4x the mass of lets say our galaxy?

Question

shamil98 · Answer

I don't know physics.. >.>

anonymous · Answer

The evidence for dark matter comes from galaxy rotation curves.

A rotation curve is simply a plot of orbital velocity as a function of distance from the centre of mass (i.e. radius).

Velocity as a function of radius $v(r)$ is related to density as a function of radius $\rho(r)$ by
$$\rho(r) = \frac{3v(r)^2}{4\pi Gr^2}$$
thus from measuring the shape of a rotation curve you can calculate the density and thus the mass of a galaxy. This disagrees with the amount of matter that is actually visible.

anonymous · Answer

Interesting, thanks for this, it had been haunting me for quite a while, but I will be doing furthermore research to verify it =P.

masumanwar · Answer

well dark matter as i see we feel only gravity of that but no electromagnetic radiation comesout is possible to be there

anonymous · Answer

@preskill89 Hey Preskill, I was hoping you could expand on what @broken_symmetry said, I've been looking for more information on this, but I can't seem to find any surprisingly, other than "yes it does exist". Also, how would one verify what broken said?

anonymous · Answer

These kinds of things keep me up at night >_<.

anonymous · Answer

Evidence of dark matter also sprouts from gravitational lensing.