Mirror matter is the correct term for anti matter. However given that the term has fallen out of use and the term antimatter is now in common use the term mirror matter has been postulated to describe a different phenomenon. For a long time now scientists have searched for the existence of massive objects to contribute to the observed mass of the Galaxy. The massive objects are not visible because they do not emit photons but their gravity bends light from background stars. The distortion of the background sphere would reveal the presence of such objects. At around half the mass of our sun they are generally considered to be too massive to be failed stars known as brown dwarfs. White dwarfs are a postulated possibility but spectrographic analyses of the areas show no sign of the heavy elements that white dwarfs would have released into space during earlier phases of their lives.
Rabindra Mohapatra of the University of Maryland at College Park. and his colleague Vigdor Teplitz have suggested that MACHOs are made of a strange kind of mirror matter generated in the big bang. Mirror matter was first suggested in the 1980s, is that every particle in nature has an elusive, unseen partner. Mirror particles would bolster theories of how the four forces in nature acted as one in the early Universe. Mirror matter would come with its own unique set of laws but would react to the force of gravity, and would therefore clump into mirror stars and planets. However, its versions of the strong, weak and electromagnetic forces would be different from those we know. Although mirror stars would burn through nuclear fusion just like normal stars, they would not emit photons, so they would be invisible.
Mohapatra and Teplitz say the case for a mirror sector is backed by data from experiments that suggest the three known neutrinos can "mix" and flip from one type to another. Some experiments hint that the three particles are mixing with a fourth kind, perhaps from the mirror world (New Scientist, 26 April 1997). Assuming that's true, the researchers have used neutrino data to calculate the strength of forces in the mirror world. From this, they predict that the maximum mass of a stable mirror star would be about half a solar mass--just right to explain the MACHOs.
The physicists, who have submitted their work to Physical Review Letters, say further experiments with neutrinos may help confirm their theory. Robert Foot of the University of Melbourne, who has independently come to similar conclusions, suggests another test. If a mirror star exploded, it would emit a burst of neutrinos. This could be detected, but the "phantom" explosion would be invisible.
"That's a possibility," says Mohapatra. However, he says there are mathematical solutions that would show that nuclear reactions would burn stellar fuel faster in the mirror world. It's possible that any mirror stars--even small ones--died young and collapsed into black holes long ago.
"They may be correct," says Charles Alcock of the Lawrence Livermore National Laboratory in California, one of a team that has discovered MACHOs. However, he thinks the idea is speculative and difficult to test. "I don't expect any of us, in our lifetimes, to be able to confirm this."
Hazel Muir From New Scientist, 13 February 1999