Below I've linked to a blog post in which Sean Carroll, a theoretical astrophysicist, describes the significance of the Bullet Cluster result. In brief: we can compare the x-rays generated by the ordinary matter to the gravitation fields reconstructed by lensing measurements. These do not match up, indicating very strongly the presence of dark matter. If dark matter were "ordinary" matter, it would have collided just like the rest of the matter in the galaxies, so we wouldn't see the gravitational field that we do. This indicates that dark matter is a different kind of matter.



Dark matter is a hypothesis that already has a significant amount of supporting evidence. There are over a dozen experiments currently running or being constructed in order to detect dark matter, so the idea that the concept is not being tested is false. There are also people who do serious work on finding alternative theories that explain the anomalies without dark matter, although most of the popular options have run into problems.

The moral of this post: don't confuse the popular presentation of science with what's actually happening in the field. These problems are very complex and there's a lot of work being done in various directions. The news media almost never come up with anything close to an accurate, thorough, or up-to-date summary, so don't rely on them for a clear picture.