“The Universe is made mostly of dark matter and dark energy, and we don’t know what either of them is.” -Saul Perlmutter
When we look out at the Solar System, the Sun dominates in terms of both light and mass. Responsible for nearly 100% of the light and for 99.8% of the mass, it stands to reason that stars would account for the vast majority of mass in the Universe. Yet when we apply what we observe of light and stars to structures like galaxies, clusters, and the large-scale structure of the Universe, not only do stars not get us there, but all the known forms of matter, including gas, dust, plasma and black holes, don’t get us there either.
In order to account for the full suite of observations as astronomers’ and cosmologists’ disposal, there has to be something more to the Universe, outmassing normal matter by a 5-to-1 ratio, than all forms of normal matter can explain. At this point in time, the only explanation that nabs them all is dark matter.