OK, I’ll give it a try to explain lensing in layman’s terms.
Essentially, light wants to be able to travel the shortest point between two points. So, for a flat sheet of paper this is a straight line; for the surface of a sphere its an arc of a circle whose centre is co-incident with the centre of the sphere (this is known as a great circle of a sphere).
One way to think about space is that its like a big stiff rubber sheet that light is confined to the surface of, and that gets deformed by masses being placed on it. If there were no masses between two points A and B, light emitted from A would travel in a straight line to B.
Now, if you stick a mass down somewhere between A and B, the shortest route between A and B is no longer a straight line, but a curve. Now, if the mass that you’d put down was a perfect sphere, then light travelling on either side of the mass would have the same travel time and you’d just see a magnified version of the light emitted from A. However, mass distributions of the size required to cause lensing rarely are spherically symmetric but tend to be distorted; thus light travelling around one side of the mass takes a different amount of time to get to us than light travelling a different route around the mass. So, we end up with distorted images of the original light source.
Now for the Bullet Cluster. A galaxy cluster consists of three main components; dark matter, galaxies and hot gas. The dark matter cannot be observed directly as it doesn’t interact in any way other than gravity, but provides the ‘potential’ that the gas and galaxies move around in. Kind of like the way the Earth provides gravity to keep the atmosphere here and to keep us on the ground. The dark matter also makes up most of the mass of a cluster.
The hot gas and galaxies, whilst they are part of the cluster, are much freer to be disturbed by other objects etc – its easier to push these things around rather than the dark matter because there is much less of it.
So, with the Bullet Cluster, what we believe has happened is that two clusters have collided, one being less massive than the other. When the clusters collided, the gas from the two clusters basically interacted with each other, heated up and essentially was greatly disturbed by the collision, because its lighter. The dark matter of the two clusters on the other hand ended up passing through each other, distorting each other gravitationally but other than that not doing much else to each other.
How does gravitational lensing fit into this then? Well, we can observe the distorted images of galaxies that are behind the bullet cluster, and using the positions of the images and the time delays between different images (i.e. looking at one image and seeing something happen and seeing how long it takes before it happens in another image), we can reconstruct where the mass that’s causing the lensing actually is. Now, because lensing is essentially a gravitational process, this shows us where the mass, i.e. the dark matter is (see above about how most of the mass in a cluster comes from the dark matter). Then, looking at the X-ray, we can see where the hot gas is, and use everything together to make inferences about the cluster.
Does this help clear things up? I can write more later, but right now, I’m supposed to be in a galaxy clusters workshop so I have to dash.