For decades, astronomers, physicists and cosmologists have theorized that the universe is filled with an exotic material called "dark matter" that explains the stranger gravitational behavior of galaxies and galaxy clusters.
Dark matter, according to mathematical models, makes up three-quarters of all the matter in the universe. But it’s never been seen or fully explained. And while dark matter has become the prevailing theory to explain one of the bigger mysteries of the universe, some scientists have looked for alternative explanations for why galaxies act the way they do.
Now, an international team of scientists says it has found new evidence that perhaps dark matter doesn’t really exist after all.
In research published in November in the Astrophysical Journal, the scientists report tiny discrepancies in the orbital speeds of distant stars that they think reveals a faint gravitational effect – and one that could put an end to the prevailing ideas of dark matter.
The study suggests an incomplete scientific understanding of gravity is behind what appears to be the gravitational strength of galaxies and galaxy clusters, rather than vast clouds of dark matter.
That might mean pure mathematics, and not invisible matter, could explain why galaxies behave as they do, said study co-author Stacy McGaugh, who heads the astronomy department at Case Western Reserve University in Cleveland.
The new research reports that signs of a faint gravitational tide, known as the “external field effect” or EFE, can be observed statistically in the orbital speeds of stars in more than 150 galaxies.
The authors say the effect cannot be explained by dark matter theories, but it’s predicted by what’s known as the modified Newtonian dynamics theory, or MOND.
“What we're really saying is that there is absolutely evidence for a discrepancy,” McGaugh said. “What you see is not what you get, if all you know about is Newton and Einstein.”
Astronomers long assumed that stars orbited the centers of galaxies at speeds predicted by the theory of gravity formulated by the English physicist and mathematician Isaac Newton more than 300 years ago.
Newton based his theory that objects attract each other with a force varying according to their mass on observations of the orbits of the planets. With refinements from the theories of the German-born physicist Albert Einstein in the 20th century, it remains astonishingly accurate.
But observations of the Coma cluster of galaxies in the 1930s by Swiss astronomer Fritz Zwicky, then working at the California Institute of Technology, found it was subject to larger-than-expected gravitational forces – an effect he attributed to “dunkel (kalt) materie,” which is German for “dark (cold) material.”
When the American astronomers Vera Rubin and Kent Ford found anomalies in the orbits of stars in galaxies in the 1970s, many scientists theorized they were caused by masses of invisible “dark matter” within and around galaxies, and the idea has dominated astrophysics ever since.
By some estimates, dark matter makes up about 85 percent of all the matter in the universe. It’s said to interact with light and visible matter only through gravity, and it explains the observed anomalies in distant galaxies.
But it’s never been seen, and so far no one has fully explained what it might be, although dark matter candidates include weakly interacting massive particles, or WIMPS, primordial black holes and neutrinos.
MOND was formulated in the 1980s by an Israeli physicist, Mordehai Milgrom, to explain the observed discrepancies without dark matter.
It proposes that gravity causes a very small acceleration, not predicted by Newton and Einstein, at such low levels that it can only be seen in galaxy-size objects; and it would mean the explanation of dark matter is not needed.
So far, MOND has survived several scientific tests – although many scientists say it cannot explain observations of the Bullet cluster of colliding galaxies, for example.
McGaugh admits that MOND is a minority view in astrophysics, and that most scientists favor the existence of dark matter – an idea he favored himself, until he began to change his mind about 25 years ago.
“I once would have said the same things: it’s absolutely proven that there’s dark matter, don’t worry about it," he said.
But many of the predictions of MOND have been seen in astronomical observations, and the latest research is one more piece of evidence for it, he said.
“MOND is the only theory that has succeeded in this way," McGaugh said. "It is the only theory that has routinely had all predictions come true.”
The new research raises “a very interesting issue,” said Matthias Bartelmann, a professor of theoretical astrophysics at Heidelberg University in Germany, who was not involved in the study.
“Can dark matter be explained by a different law of gravity? It would be most important for cosmology as well as particle physics if it could," he said in an email.
He has doubts, however, that the “external field effect” reported in the new research is truly a unique prediction of MOND, and that it cannot be explained by some competing theories.
And since MOND theory was formulated to account for the rotational discrepancies in galaxies, testing it on galaxies would be expected to return convincing results; instead, MOND needed to be tested successfully on other objects, such as galaxy clusters, he said.