In 1981 Milgrom began working on papers to publicize his idea. He was laboring alone—since returning from Princeton, he had told no one of his work but his wife. He sent drafts of his papers to a few mentors and colleagues. Their reactions were muted but encouraging. "None of them reacted violently," Milgrom says. "I even got some helpful comments."

In spite of the suggestions Milgrom had received—all from world-class scientists—getting the papers published became an ordeal. "I was a little naive," Milgrom says. "I thought the papers would be welcomed. They were rejected by the journals at first. The reasons varied: 'It was all nonsense'; 'It's too early to consider an alternative to Newton'; 'There is no trouble yet; the flat rotational curves will be resolved in other ways.' "

Looking back on this period, Milgrom betrays no bitterness. "I went back and looked at the history of science and saw this happens again and again. The marketplace can only handle so many heretical ideas at one time. I think on the whole I have been treated fairly." After Milgrom's dogged persistence, all three of his original papers on modified Newtonian dynamics were published side by side in 1983 in Volume 270 of The Astrophysical Journal, a premier publication in the field.


As is often the case with radical ideas, the community's reaction was not scorn but silence. "At first the work was not accepted, not even really looked at," Milgrom recalls. By this time he had begun collaborating on MOND with fellow Israeli theorist Jacob Bekenstein. Bekenstein, who had already won some acclaim for his work on black holes, became a hard-core MONDista. "In 1986 we were invited to present a talk at a meeting in Princeton," says Milgrom. "This made us really happy. At least we were getting noticed." MOND began to make inroads. Its solution to the galaxy-rotation-curve problem was too elegant to ignore. For most galaxies it explained observations better than dark matter did.

But why did MOND work? What was the justification for changing Newton's law other than that it made the rotation-curve problem disappear? There was no reason, and Milgrom knew it. His solution wasn't a theory; it was simply a description and did not explain anything from first principles. Meanwhile, the dark-matter hypothesis had become ever more sophisticated. So while Milgrom and a handful of true believers continued to work on MOND, dark matter had attracted legions of supporters and became the subject of hundreds of research papers.

Where dark matter once seemed as ad hoc as Milgrom's proposal, it had over the past decade morphed into a full-blown theory that explained far more than just the peculiar movement of stars in galaxies. Dark matter had become crucial to understanding the entire large-scale structure of the universe and how galaxies formed in the first place. One of the most striking astronomical discoveries of the past 20 years is that galaxies are not randomly scattered. They're organized in vast sheets hundreds of millions of light-years in extent. Huge expanses devoid of visible matter separate the sheets. The only explanation cosmologists can offer for this structure is that the enormous galactic sheets must themselves be embedded in even larger agglomerations of dark matter.

This image shows 11,000 galaxies, depicted as dots, with the Milky Way at the center. Cosmologists typically attribute the latticelike distribution to the pull of dark matter.

Supporters of dark matter draw their most convincing evidence from the early universe. For the past few years, a NASA spacecraft called the Wilkinson Microwave Anisotropy Probe, or WMAP, has been studying the cosmic microwave background radiation, which is a relic of the Big Bang. The fine details of the radiation hold clues about how matter was distributed when the universe was only a few hundred thousand years old. Dark-matter models have predicted what WMAP has seen with such stunning accuracy that cosmologists now rely on dark matter to explain the entire evolution of the universe.

"The big difference between now and 20 years ago," James Peebles says, "is the quality of cosmological data [from WMAP]. I am deeply impressed with the way dark matter has explained cosmological observations."

To make MOND a serious alternative to dark matter, Milgrom's inspired guess needed to mature into a true theory, with a firm foundation in modern physics. And that meant confronting not just Newton but his wild-haired offspring, Einstein. It was Einstein who divined the interconnections between gravity, space, and time. For MOND to make headway in the field, someone was going to have to find a way to reconcile it with Einstein's masterpiece, the theory of general relativity.

"You can compare the first version of MOND to Kepler discovering the shape of planetary orbits," says Mario Livio, the senior astronomer at the Space Telescope Science Institute in Baltimore. In 1605 Kepler figured out that planets have elliptical orbits, but he could not explain why. It took the genius of Newton, in his 1687 Principia Mathematica, to finish what Kepler started and provide a complete theory, including the nature of the gravitational force.

Milgrom needed to do the same for MOND to expand its explanatory power. "Of course I knew a relativistic version of MOND was needed," he says, "but, hey, there is no theory of quantum gravity yet either. MOND had survived so many other tests over the years that my confidence had grown with time. There has never been anything drastically wrong found with the MOND framework. It simply was not equipped to deal with cosmology and galaxy formation."