I was in the same camp as you until the results from the Bullet Cluster were published (https://arstechnica.com/science/2006/08/5058/ https://arstechnica.com/sc

I was in the same camp as you until the results from the Bullet Cluster were published (https://arstechnica.com/science/2006/08/5058/ https://arstechnica.com/science/2006/08/5058/). Essentially, for the first time astronomers were able to observe the separation of visible and dark matter. In other words, there was evidence for a gravitational force where no visible matter existed. That said, I think you're wrong about alternate hypotheses being quickly dismissed. Alternatives are regularly proposed and evaluated. Many are eventually found to be lacking in the face of the evidence collected, but some remain viable candidates (for example: https://arstechnica.com/science/2017/01/new-ideas-on-gravity-would-vanquish-dark-matter/ https://arstechnica.com/science/2017/01/new-ideas-on-gravity...).