WASHINGTON -- The way University of Utah professor James W. Cronin describes it, he won America's top science prize Tuesday for helping the world realize it does not know the answer to a very big, basic question -- and for leading efforts to find it.

"We do not know what this universe is made of," Cronin said shortly before President Clinton awarded him the National Medal of Science, often called the "U.S. Nobel Prize."Cronin, who also won an actual Nobel Prize for physics in 1980, said scientists are finding that the lion's share of matter scattered in the vast reaches of the universe is of an unknown nature.

"It's a kind of matter that is not what we are built of -- we almost know that for sure," and his work on cosmic rays shows it acts differently than old theories have predicted, he said.

Cronin said discovering the secrets of such matter may be possible with technological advances in the next 25 years and is leading an international consortium seeking to build cosmic ray observatories in Argentina and Utah's Millard County to help find such answers.

Cronin's cosmic ray research, plus earlier Nobel Prize-winning work on matter and antimatter, led him to win one of two National Science Medals awarded for physical sciences this year. Ten other medals were awarded in other branches of science.

Only 386 people have been awarded a Medal of Science since the program was established by Congress in 1959. The National Science Foundation screens nominees and sends recommendations to the president for selection.

President Clinton told winners Tuesday, "The work that you and your colleagues have done has changed everything about our lives. It has brought us to the threshold of a new scientific voyage that promises to change everything all over again."

Cronin said one such voyage is untangling riddles of cosmic rays that hit Earth with such high intensity that they seem to defy known laws and theories of science. They may unlock mysteries about the unknown nature of vast amounts of matter scattered in the universe.

"What do you expect in the next 25 years? Certainly we will know what that (unknown matter) is, and whether it is useful or not. Who's so clairvoyant to know that it won't have some value?" Cronin said.

Cronin is a part-time professor at the U. and a professor emeritus at the University of Chicago. He is on a one-year leave from the U. to teach in Paris.

Cronin said he came to Utah to be near work at a current cosmic ray observatory in Dugway and a much larger planned observatory in Millard County.

"We have some difficulties in coming to some good agreement among various parties in the United States" to build the proposed Millard County observatory, he said.

"However, we are building the (sister) cosmic ray facility in Argentina right now," he said. The two observatories -- one in the northern hemisphere and one in the southern -- are jointly called the Pierre Auger Observatory Project.

Cronin predicted the proposed observatory in Millard County, which would consist of tanks of water scatter over 1,600 square miles to serve as cosmic ray detectors, will be built eventually despite current problems.

"Sooner or later it will appear in Millard County in one form or another. That's why I keep my association with Utah," he said.

Cronin said scientists are getting closer to answering big questions about the universe. "All this technology we talk about gives us instruments where sooner or later we're going to find out what the nature is of this missing matter, or unknown matter."