There have been many ideas, it depends on which component of 'missing mass' you are interested in. It seems that the halos of many galaxies have non luminous 'stuff' that contributes to the weight of a galaxy.
Theory 1: says that this stuff is sub-luminous brown dwarfs, and many have searched for light from this population and not found any.
Theory 2: says that the halo missing mass is in the form of neutron stars ejected from the disk of the Milky Way during supernovae explosions. Many pulsars have been detected with velocities approaching 1000 kilometers per second, so it is possible that over billions of years the halo could be filled with these, but no one seems to understand how enough could have been ejected from the plane of the Milky Way to allow 2- 3 time more MASS to now be in the halo than in the plane of the Milky Way! Searches have been made for gravitational lensing events by so-called MACHOS which are Massive Compact Halo Objects including neutron stars and black holes. There could be huge numbers of black holes in the halo left over from the first billion years of star forming activity in the Milky Way where the stars were much more massive than today's populations, and all are thought to have evolved into supernovae and black holes after a few 100 million years. About a dozen MACHO lensing events have been detected by monitoring millions of stars in the Large Magellanic Cloud. This approach looks promising, but more research is needed to improve the estimates for how much halo mass this translates into.
Theory 3 says that the galactic haloes are not made of baryonic matter ( even collapsed matter in neutron stars and black holes!) but is in a more exotic particle such as a massive neutrino, or one of the many particles predicted by supersymmetry theory in theoretical physics. If one of the ordinary neutrinos had a mass of as little as 17 electron volts, 17/512,000 the mass of an electron, there would be enough of them to give the halo of the Milky Way a very high mass. Some recent experiments, however, suggest a neutrino mass much lower than this.
Now, if you are talking about cosmological missing mass, you have to include the fact that some clusters of galaxies show that the galaxies are embedded in a very hot, million degree plasma, and that many clusters would not be stable against dissipation unless there was 2- 5 times more gravitating mass than what you can see in the visible stars and hot plasma.
Theory 1 says that the universe is not closed or 'critical, so that the mass you see is all that there is. The baryonic universe contains the vast majority of the gravitating stuff in the universe and so the universe is 'open' and has 1/10 the critical density for collapse. This theory is, as of 2000, no longer considered to be consistent with the bulk of the new data now available.
Theory 2 says that the universe is at its critical density, and the missing mass is being provided by some new 'non-baryonic' hot/lukewarm material such as massive neutrinos or other exotic particles. There is no evidence for the conventional neutrinos having more than enough mass to provide 20 percent of the critical density. So the other 80 percent has to be an exotic particle we haven't discovered yet in our atom-smashers. As of 2000, it is no longer considered plausible that Dark Matter can provide all of the Omega=1 required by this model.
Theory 3 says that among the ingredients to the universe is a 'non-zero' cosmological constant provided by a cosmological scalar field that permeates all of space. As of 2000, this is the theory of choice because the available data after 1998 strongly indicates from two different sets of studies that the universe is accelerating its expansion. The best estimates are that the cosmological constant accounts for Omega = 0.70 and dark matter plus luminous matter accounts for about 0.30. See the above figure for details.
There are other theories about 'hot dark matter' versus 'cold dark matter' but these are mostly refinements of Theory 2 in the cosmology section. We still have no idea what this dark matter is!
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.