Since the force of gravity relates precisely to the mass of an object, we can estimate the mass of the universe by observing the forces of gravity in the universe.
We also can add up the known mass of different atoms and particles in the universe.
The problem is that when astronomers add up the mass of all the objects they can see and all the objects they can’t see but know exist, they still come up short.
We can only account for about 6 or 7 percent of the mass we know is needed for the universe to behave the way it does.
The mass we haven’t yet found is called dark matter.