So, as promised, I have more dark matter evidence to tell you about! So you hear about Einstein and his many contributions all the time. One of those is general relativity. Relax! No math today, just a brief overview. The math scares even me, don't worry. (But it is cool!)
GR is a theory of gravity that says gravitational interactions happen because masses warp space-time around them. Here's the type of picture we always use to visualize that, space-time warping near a massive object: 
In our daily life, we don't encounter massive enough things for this to really matter. But for very massive entities, space-time warps heavily around them. Now, light travels in straight lines. But if space-time ITSELF is curved? The straight line can look like a bend.
The same way the lenses in a pair of glasses manipulate light to create optical effects, massive bodies act like a lens due to gravity. We call this gravitational lensing! Below is a NASA/ESA image of the Einstein Cross, four images of the same object being lensed by a galaxy:
Returning to dark matter being a discrepancy between mass that produces light and total mass exerting gravity, you can apply that here too! If there is mass that isn't producing light, when you analyze the lensing effect it will imply a lens with greater mass than what you see!
