The Importance of Unbiased Mass Mapping

There's a lot of mathematical theorems that goes into this. If you think about two objects in different directions from the source of mass, they're going to be lens differently. So getting into mass mapping is more complicated than we can really do here. But the end result is that people did this for the bullet cluster and it came out to be pretty clearly that the mass was concentrated in the same location as where all the stars were. This could only really be explained by there being a lot of dark matter there, which is much more massive than the gas in the center.