Most of the mass in the universe isn't a type of matter we are familiar with. Instead, it's a mysterious kind of “dark matter” that doesn't emit or absorb light. It also interacts rarely both with itself and with normal matter, too rarely to have left any trace in our detectors.
We know dark matter is out there because we see its gravitational pull. Without dark matter, Einstein's theory of general relativity does not predict a universe that looks like what we see; neither galaxies, nor galaxy clusters, nor galactic filaments come out right. At least that's what I used to think.