"Dark matter" is a theoretical gap filler, partly to attempt to explain observations, as you say, but also to try to save the expanding universe theory, to which you allude. I don't think you are doing that in this article, but it is worth noting that theoretical speculative metaphysics. https://medium.com/inserting-philosophy/what-if-the-universe-is-not-expanding-7a6d3d6a2d2e