“The discrepancy between what was expected and what has been observed has grown over the years, and we’re straining harder and harder to fill the gap.” –Jeremiah P. Ostriker
When it comes to the structure of the Universe — forming the galaxies, clusters, and Universe as we see it — the normal matter we know of simply isn’t enough. Given our best-understood laws of physics, including Einstein’s general relativity, what we see of galaxies and the Universe in general simply doesn’t match up to our predictions.
The simplest solution, arguably, is to just add a new ingredient: a new form of matter, a dark matter if you will. But a counterargument is that we’ve got the laws of gravity wrong, and that no new matter is necessary. There’s only one way to settle an argument like this: with data, evidence and the full suite of observations at our disposal.