In 1933 Fritz Zwicky was studying the Coma galaxy cluster.  He wanted to know the total mass of the cluster, but using the velocities of the galaxies and the viral method resulted in a mass far greater than what could be accounted for visually.  So, to solve this problem he introduced the concept of dark matter and defined it as matter that cannot be directly detected by its own emitted radiation. Now that dark matter is accepted, we assume that it exists everywhere, but we typically associate it with galaxies in the form of dark matter halos, in order to explain galaxy kinematics such as the flattening out of the rotation curves or even why galaxies stay together.