There are a couple of ways to measure the mass of the universe. The second method is to measure the velocities of stars in spiral galaxies and calculate how much mass is required to create the gravity that causes those motions. This measurement shows that there is a lot of non-shining matter, called dark matter. Dark matter is a hot topic of research. This is where the particle physicists and crackpots hop on board.
Neutrinos are a favorite candidate.
We know that lots of neutrinos exist, but so far we have considered them to be massless. Even if they have a teeny weeny mass, there are enough of them to contribute to the dark matter. Other exotic particles, like axions and neutralinos, which were proposed for other reasons, may contribute to the dark matter. But in general, all you need is a particle that hasn't been seen before, but that solves all your dark matter problems. Lots of wacky candidates have been proposed and at this point it becomes difficult to distinguish the serious, studied proposals from astroparticle geniuses and the incoherent rantings of crackpots.