90% of mass in a galaxy is non-luminous matter, meaning matter that does not give off light and thus we cannot see. It is called dark matter.
We are confident that dark matter exists for a number of reasons. The best experimental reason is the flat orbital velocity curve.
We can measure the velocity of stars in nearby galaxies by measuring the Doppler shift of their spectra. If we plot the velocity of stars as a function of their distance from the center of the galaxy, we notice that the curve is flat, as shown below.
The only explanation for this graph is that there is much more mass throughout the galaxy than what we actually see. Present estimates are that 90% of the total mass in a galaxy is dark matter.
What is dark matter? Perhaps it is made up of stellar remnants such as black holes, neutron stars, and white dwarfs. Or perhaps it is not ordinary matter at all but made up of types of particles that we haven't yet discovered.
Another reason that we are confident in the existence of dark matter in galaxies is that mass bends light, an effect called gravitational lensing. General relativity is used to determine the mass of a galaxy based on gravitational lensing, and this confirms the large amount of non-luninous matter in galaxies.