Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see?

Since the late1990s, a consensus has emerged that some sort of "cold dark matter" is needed to explain all sorts of things we see.  For example, in2003 an analysis of data from the Wilkinson Microwave Anisotropy Probe suggested that the energy density of the universe consists of about23% cold dark matter, as compared to only4% ordinary matter.  (The rest is dark energy.)

Unfortunately nobody knows what this cold dark matter is!  It probably can't be ordinary matter we've neglected, or neutrinos, since these wouldn't have been sufficiently "cold" in the early universe to collapse into the lumps needed for galaxy formation.  There are many theories about what it might be.  There's also still a possibility that we are very confused about something, like our theory of gravity.