In Part One we discussed how an image of a galaxy can be 'photocopied' multiple times on the sky. This bizarre idea calls to mind the common quote that 'fact is stranger than fiction.' This phenomenon is a kind of optical illusion that happens only every once in a while when light shining from a very distant passes through a natural eyeglass in space called a gravitational lens.
What is this lens that produces such grand optical illusions? The lens is not made of glass, or some kind of high-index plastic. This is the interesting part: instead, the lens is made out of matter. Think of it this way: when a person obstructs your view of another person, then you lose track of that second person as one is not able to see through humans.
Curiously, when a huge amount of matter obstructs your view of another object, the outcome is different. In this case the light from the more distant object will still manage to make it to the telescope. This happens because the huge amount of matter, the lens, deflects the light around it. The lens makes the light turn corners. And unlike optical lenses, gravitational lenses can be huge, much bigger than the Earth and in fact much bigger than the Milky Way! Such lenses can make the light from entire galaxies appear to `turn corners.'
We can estimate the total weight of these gigantic lenses. When we do that, we find that the lenses weigh less than they should given the amount by which they are bending the light around them. This is puzzling. Are there some galaxies we are forgetting to include in our tally that are so faint that we cannot detect them? All of this missing matter which we cannot see in the lens and cannot account for in any other way we call the dark matter. Yikes - we still do not know what is this mysterious other matter which we are not made of and yet seems to be present nearly everywhere in the universe.