Recently, astronomers have reported the observation of what they think of as dark matter in two colliding galaxies. They used observations from two sources: the Chandra X-ray observatory for the distribution of visible matter, and a gravitational lense for the distribution of other---dark---matter. Lucky observations, brilliant deduction. What I am curious about is how people incorporate dark matter into general relativity. The material is hypothesized to be detectable through it gravitational effect only, but in order to make meaningful calculations (such as the location and size of the dark matter blobs), you need to have some idea of the magnitude, right? In addition, the lensing effect is a consequence of curved spacetime, so how was the strength of that lense established? Am I correct in assuming that you don't need to know what actually causes the curvature in order to use its effect? (Otherwise you'd have to take into account the possibility that the lense itself is composed of dark matter which you don't know much about, etcetera.) Please bear in mind that I have no background in general relativity beyond a very qualitative description of what it is and does---most engineers don't really need general relativity on a daily basis. If my questions seem odd or obvious, then you understand why .