“For the moment we might very well can them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere).” -Bill Bryson
The Sun makes up 99.8% the mass of our Solar System, yet stars account for only about 10-20% of the matter that protons, neutrons and electrons make up. Protons, neutrons and electrons — along with all the other particles known to exist, represented by the Standard Model and what it builds — make up only about 15% of the observed matter. The remainder must be something different that doesn’t interact with electromagnetism or light: dark matter.
That’s the conventional picture. But must that be the case? Is it possible, as an alternative, that we’ve simply got an incomplete theory of gravity, and that there’s no such thing as dark matter after all? While there’s one piece of evidence that supports that possibility, there are a great many other lines of evidence that completely discount it, leaving dark matter as the only viable alternative thus far.