In my physics research project on dark energy I touched on dark matter - recently I learned more about evidence for it (you can thank the University of Lancaster) so here is a little piece about that.
Dark matter is the mysterious 85% of all matter that we think exists but barely know anything about. It only interacts with ‘ordinary’ matter in one way - it has mass. Knowing that, it seems like it should be easy check if it is there - we can use the laws of gravity to figure out if it is present because other things we can see are attracted to it.
We do see a difference between how much mass we expect to be in an area and how much we can calculate there is based on gravity, however this itself does not prove that dark matter exists. This is because it is possible that we have just got the gravity equation wrong when it is applied to a large scale. This is called a modified gravity theory.
Luckily, gravity is not just any force, it is the distortion of spacetime. As well as gravitational attraction, it causes gravitational lensing, where mass can cause light to bend, creating characteristic curved shapes. If dark matter is real then it should cause it.
The problem that we have here is that dark matter is almost always found mixed in with regular matter - we need it to separate to see if it is there. You might now see how colliding galaxies fit in here.
Have a look at this picture of the bullet cluster galaxy. Two galaxies have smashed head on into each other, the visible mass colliding and slowing down (this is shown in pink). However the dark matter has not collided - dark matter does not interact other than via gravity so it has continued on its path largely unchanged.
We can use gravitational lensing to show that the mass of the combined galaxy (shown in blue) is more spread out than expected from the visible matter. This is the best evidence that we have for dark matter - and why it is the most popular explanation among scientists.