Introduction:
A galaxy rotation curve is a plot of the orbital velocities of stars or gas in the observed galaxy versus the radial distance from the galaxy's center. What turns out is that galaxies in our universe seem to achieve too high velocities, such that the gravity generated by the observable matter (stars, gas) could not hold them together. This has led scientists to believe that there is some extra matter, not visible, that generates the extra gravity that galaxies need to stay intact. This extra matter is what we call dark matter, one of the most fascinating problems in physics nowadays. Unlike normal matter, dark matter does not interact with the electromagnetic force; therefore it does not absorb, emit or reflect light and because of this it is very hard to detect. It is possible that it interacts through gravity and weak force (WIMPs-weakly interacting massive particles), but it is also possible that it interacts only through gravity, which would make it even harder to spot.
There are also other indications for the presence of this new type of non-baryonic matter, maybe the most famous is the Bullet Cluster, two colliding clusters of galaxies with an evidence of a split between visible matter and dark matter. However, it should be noted that there are alternative theories, which try to justify the observations, like the ones that modify the laws of gravity.
