ASTRONOMERS knew little about the overall structure and substance of the universe when the 20th century began. By the 1950s, they thought they grasped its essence. They believed the cosmos to be uniform on the large scale so that any one part of it would look the same as any other part. They believed it to consist primarily of hydrogen - the principal element by which stars and galaxies shine. But as the 20th century nears its end, they confront their earlier ignorance. Recent discoveries suggest the universe has lumps and bubbles of mammoth scale. Last fall, R.Brent Tully of the University of Hawaii reported finding what may be the largest such irregularity yet known. It's an association of galaxy clusters a billion light-years long. A light-year, or the distance light travels in a year, is 6 million, million miles. It would take light a billion years - roughly one-tenth of the age of the universe - to travel the length of this structure.
If the universe is homogeneous in the large, astronomers have yet to prove it. Dr. Tully notes that observers expected to begin to see this smoothness as their studies embraced larger volumes. Instead, he says, ``We see more inhomogeneity.''
Also, hydrogen no longer seems to be the primary cosmic substance. A great deal of mass - some experts suggest as much as 90 percent of the universe - is invisible. Astronomers detect this so-called dark mass - when they can do so at all - only by gravitational influence. Some theorists believe the bulk of it has yet to be found and that it may exist as new forms of matter.
Thus astronomers once again must confess they don't know what the universe is made of. Referring to this burgeoning ignorance, Yale University astrophysicist Lawrence M. Krauss has observed that ``cosmology is in many ways in its infancy.''
As astronomers wrestle with this ancient question of what our universe is really like, they point out that they expect some clumpiness. It's the vast scale of these irregularities that surprises them.
A completely smoothed out and featureless universe would be unstable. Small differences in density would develop. The denser clumps would grow as their stronger gravity pulled in material from their surroundings. Indeed, galaxies, clusters of galaxies, and gravitationally bound associations of clusters called superclusters may have formed at least partly in this way. But astronomers expected such structures to be more or less randomly distributed. Instead, they are finding a kind of Swiss cheese or bubble-bath arrangement. Vast associations of galaxies seem to spread around the bubble walls, leaving relatively empty voids inside.
Five years ago, Tully was pointing out that evidence available at that time showed that ``clumps and holes can exist on scales as large as 300 million light-years.'' Shortly thereafter, Riccardo Giovanelli of the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico, and Martha P. Haynes of the National Radio Astronomy Observatory at Green Bank, W.Va., mapped part of the sky at radio wavelengths and found a band of associated galaxies 700 million light-years long. It was the largest continuous structure charted on the sky up to that time. Dust had hidden it in surveys made with optical telescopes. But it showed up in the radio maps as a continuous band of galaxies linking two previously known superclusters - one in the constellations Perseus and Pisces and the other in Ursa Major and Lynx.
Then, about two years ago, John Huchra, Margaret Geller, and Valerie d'e Lapparent of the Harvard-Smithsonian Center for Astrophysics made a three-dimensional map of a slice of sky that contains nearly 1,100 galaxies. In the angular measure astronomers use in sky mapping, the slice was 6 degrees thick by 117 degrees wide and extended 450 to 900 million light-years in depth. Although that's a small part of the cosmos, it is big enough to suggest how galaxies are distributed. Instead of the expected random scattering, galaxies appeared to lie on the surface of voids 90 to 150 million light-years in diameter. They appeared like flecks of foam on the surface of soap bubbles.
Astronomers had seen that sort of thing before but thought it a rare occurrence. Continuing work by Dr. Huchra and associates and by other investigators has shown the pattern to be common. Most galaxies seem to gather around the edges of ``bubbles'' up to 450 million light-years across with little luminous matter inside. Seen in two dimensions, such an arrangement would give the impression of galaxy chains stretching like filaments across the sky. It's a degree of organized structuring on a surprisingly large scale for a universe whose material was supposed to be homogeneous and randomly distributed when taken in sizable volumes.
Such was the situation last November when Tully announced the discovery of organized structure on an even grander scale. This supercluster complex, as he calls it, lies in the constellations Pisces and Cetus. It encompasses millions of galaxies and stretches something like one-tenth of the way across the observable universe. Tully says he has preliminary evidence of four other such supercluster complexes. But he can't fully investigate them, because they stretch beyond the volume of space that has been surveyed. ``There are no good limits on how large they might be,'' he explains.
There are other indications of large-scale structure or lumpiness. Over the past decade astronomers have convinced themselves that the Virgo supercluster, in which our Milky Way galaxy is embedded, and a couple of nearby superclusters are sliding in the direction of the Southern Cross. This slip, at some million miles an hour, may be due to the gravitational pull of an as yet unobserved monster mass that astronomers call the Great Attracter. If it exists, it would be perhaps several million light-years across by several hundred thousand light-years wide. And it might well consist, at least in part, of some of the mysterious dark matter whose shadowy presence now intrigues astronomers.
Matter within galaxies and galaxies within clusters move much too fast to be held together by the gravitational pull of their visible stars, gas, and dust alone. Here ``visible'' means detectable by any kind of electromagnetic radiation including infrared, light, radio, X-ray, and the like. It's as though some unseen material, whose mass exceeds the visible mass by several times, must also be present.
Also, for complex theoretical reasons, many cosmologists think there should be enough mass to provide the gravity to eventually halt the expansion of the universe. It has been expanding since it was created in a primordial ``big bang'' explosion of energy some 10 to 20 billion years ago, according to modern cosmological theory. The visible material provides only about 10 percent of the needed mass.
Thus astronomers and cosmologists think as much as 90 percent of the mass of the universe may consist of the dark matter. But they don't know what that is. Dead stars and other forms of nonluminous ordinary matter wouldn't be plentiful enough to account for it.
Among the possibilities are certain hypothetical particles predicted by theories that try to unify the basic natural forces as different aspects of a single underlying force. Physicists call them weakly interacting massive particles, or WIMPs. (If they interacted strongly with conventional matter, they would already have been observed.) The big bang would have produced them in abundance if they can exist. It would also have produced an abundance of even more exotic speculative objects that theorists call cosmic strings. Such strings would be dense energy concentrations infinitesimally thin, yet many millions of light-years long and extremely massive. But until some evidence for WIMPs and strings is found, the nature and extent of their possible contribution to the dark matter is only conjecture.
Meanwhile, cosmologists aren't yet giving up their faith in the homogeneity of the universe, when viewed as a whole. They point to the low-frequency radio energy, left over from the big bang, that permeates the cosmos. It seems highly uniform across the entire sky. Also, inhomogeneity on the scale of Tully's proposed structure is not yet fully proved. As Tully himself admits, what he takes to be a giant association of galaxies may merely be an artifact produced by inaccurate and incomplete observations. It may vanish when more detailed sky surveys are made.
But on one point all cosmological experts agree. The universe has far more structure and material in it than anyone imagined a couple of decades ago.