By measuring the speeds of stars within galaxies, and the speeds of galaxies within distant clusters of galaxies, astronomers have become convinced over the decades that there is a sub-luminous or even non-luminous 'something' which contributes to the gravitational fields of these systems, but which you cannot count-up optically so that the speeds and the masses 'balance' each other. This is 'dark matter' or 'missing mass', and some physicists have proposed it may not even be matter. The first kind of dark matter we know about is just faint stars, distant white dwarfs, neutron stars and black holes. These are, or once were, all made of ordinary matter (protons and neutrons) and have already been tallied in the weighing of the matter content of the cosmos. Next come neutrinos, which are known to carry a little mass but not enough to amount to more than a few percent of the dark matter. Finally, we have to deal with the unknown kind of dark matter that dominates the cosmos completely. We have no clue what this stuff is; only that it is probably very 'cold' which means it is massive and slow-moving. In the ancient universe, it was cold enough that it formed gravity wells that the hotter, ordinary, matter eventually fell into to start clumping to form galaxies. Physicists have not seen any particle that would fit this new material, but imagine several candidates called axions, gravitinos or simply 'neutralinos'. Until we see some traces of this stuff in our ground-based accelerator labs at Fermilab
