Despite centuries of searching, astronomers still wonder where most of the matter in the universe is hiding. They don't even know what form of matter it may be.
Recent searches for the elusive material have not yet located it. But they have narrowed the possibilities astronomers must consider.
The puzzle arises because such indirect indicators as the motion of galaxies and clusters of galaxies show there is much more matter present in the universe than all the stars, dust, and gas now known can account for. This is true of the universe at large and on the much smaller scale of our own stellar neighborhood - the region within a few hundred light-years of the sun.
Solving this cosmic puzzle is important for understanding the makeup of the universe, and of our own local cosmic neighborhood. It is also critical for judging whether the universe will go on expanding forever or whether there is enough matter to supply the gravity to halt that expansion and even reverse it.
Gerard Gilmore of the Royal Observatory in Edinburgh and Paul Hewett of the Institute of Astronomy at Cambridge University, England, have taken one of the latest looks at this problem. As they note in reporting their work in Nature, ''no plausible candidate for this substance (the unaccounted-for matter) is known.'' Many astronomers, however, have speculated that very dim, hard-to-see stars of a type known as M dwarfs might account for at least some of it. Gilmore and Hewett have made a new search for such stars and can't find many of them. This, they say, narrows the possibilities as to what the missing matter may be.
They say their survey is thorough enough to show that dim stars can't account for the long-sought material within the solar neighborhood. They are also unlikely to account for it in the universe at large. Whatever that elusive, invisible mass is, it is more exotic than M dwarf stars.
Findings such as these drive astronomers to extreme speculation. They wonder if particles such as the gravitron - which has yet to be observed - may be partly responsible. Gravitrons would be particles associated with gravitational fields. They speculate that neutrinos - particles that are well known and have never been found to have any mass - may have some residual mass after all. Even very lightweight neutrinos might be numerous enough to account for the missing mass.
But so far the only definite evidence concerning this mass has come from surveys, such as that of Gilmore and Hewett, which show what it cannot be, not what it is.
Meanwhile, on the larger question of whether there is enough mass - missing or not - to halt the universe's expansion, a recent study indicates the universe will go on expanding forever. An Australian-British-Chinese team has made what it considers the most accurate estimate yet of the mass density of the universe. This is too low to hold the cosmos together, according to team spokesman Bruce Patterson of the Australian National University.