I curious, isn't the term 'dark matter' just a modern way to say 'aether'? I mean, a long time ago people thought that space was composed of aether because they didn't know what space was comprised of and needed a label with which to call the unknown unseen 'stuff'. Now they're saying space is full of 'dark matter' because it is thought there has to be something and they need a label to call this unknown unseen 'stuff'. So why not just call it aether? I just think it is kind of funny because it looks astrophysicists (or are you called cosmologists?) have come full circle back to aether.
The idea of the aether came about because it was believed that light behaved the way that waves of water or sound do - that is that light needed to travel through a medium. Just like waves must propagate through water and sound must propagate through air, so too light had to propagate through aether. And it was a perfectly reasonable theory until it was proven to be wrong.
So you are right in the sense that dark matter is a label which is being applied to something we don't completely understand in much the same way that aether was. But what invalidated the theory of aether was the Michelson-Morley experiment which tested an important prediction of the aether theory. Since Earth moves around the Sun and the Sun moves around the center of the Galaxy, it should have been possible for Michelson and Morley to detect an "aether wind" with their experiment as Earth moved through the aether. Instead, Michelson and Morley found nothing or at least not enough to justify the existence of an aether. And thus the aether theory faded into history as a failed theory.
The modern equivalent of the Michelson-Morley experiment for dark matter would be if there were some way to measure the strength of gravity (and thus the amount of mass) in a galaxy or cluster of galaxies. And in fact we do have a way to do that through gravitational lensing in which the light of background galaxies is distorted by nearby galaxies. So if we could find a region of space with a large cluster of galaxies distorting the light from other background galaxies and if there was a great deal of gravitational lensing coming from a region of the cluster are which has no visible galaxies, it would be considered fairly compelling evidence that dark matter exists. And that's why people keep bringing up the Bullet Cluster. Because the Bullet Cluster is just such a galaxy cluster where most of the gravity and thus most of the mass can be detected outside of the visible galaxies of the cluster. And while this isn't conclusive proof of dark matter's existence, it is a fairly compelling piece of data and since the Bullet Cluster was discovered there have been other similar observations which make the existence of dark matter seem more likely. So unlike the aether which failed its experimental test, dark matter has for now passed its test and continues as a viable theory.
Any alternative to dark matter would have to explain phenomena like the Bullet Cluster at least as well as dark matter does in much the same way that Einstein's Theory of Relativity simplifies into Newtonian physics when we deal with speeds significantly slower than that of light.