What is dark matter? It has never been observed, yet scientists estimate that it makes up 85% of the matter in the universe. The short answer is that no one knows what dark matter is. More than a century ago, Lord Kelvin offered it as an explanation for the velocity of stars in our own galaxy. Decades later, Swedish astronomer Knut Lundmark noted that the universe must contain much more matter than we can observe. Scientists since the 1960s and ’70s have been trying to figure out what this mysterious substance is, using ever-more complicated technology. However, a growing number of physicists suspect that the answer may be that there is no such thing as dark matter at all.
Scientists can observe far-away matter in a number of ways. Equipment such as the famous Hubble telescope measures visible light while other technology, such as radio telescopes, measures non-visible phenomena. Scientists often spend years gathering data and then proceed to analyze it to make the most sense of what they are seeing.
What became abundantly clear as more and more data came in was that galaxies were not behaving as expected. The stars at the outer edges of some galaxies were moving far too fast. Galaxies are held together by the force of gravity, which is strongest at the center where most of the mass is. Stars at the outer edges of disk galaxies were moving so fast that the force of gravity generated by the observable matter there wouldn’t have been able to keep them from flying out into deep space.
Scientists thought that there must be more matter present in these galaxies than we can currently observe. Something must be keeping the stars from flying away, and they called that something dark matter. They couldn’t really say what properties it might have except that it must have gravitational pull, and there must be quite a bit of it. In fact, the vast majority of the universe (a whopping 85%) must be dark matter. Otherwise, galaxies wouldn’t have been able to stick around as long as they seem to do. They would have broken up because there wouldn’t have been enough gravity to keep the trillions of stars in place.
When it comes to science, the trouble with something that you can’t observe is that it’s hard to say much about it. Because dark matter does not interact with the electromagnetic force — which is responsible for visible light, radio waves, and x-rays — all of our evidence is indirect. Scientists have been trying to figure out ways to observe dark matter and make predictions based on theories of it but without much success.
A Possible Solution
Newton’s Theory of Gravity explains most large-scale events fairly well. Everything from throwing the first pitch at a Yankees game to the movements of constellations can be explained using Newton’s theory. However, the theory is not foolproof. Einstein’s theories of general and special relativity, for example, explained data that Newton’s theory couldn’t. Scientists still use Newton’s theory because it works in the overwhelming majority of cases and has much simpler equations.
Dark matter was proposed as a way to reconcile Newtonian physics with the data. But what if, instead of reconciliation, a modified theory is needed. This is where an Israeli physicist named Mordehai Milgrom makes an entrance. He developed a theory of gravity (called Modified Newtonian Dynamics or “Mond” for short) in 1982 that postulates gravity functions differently when it becomes very weak, such as at the edge of disk galaxies.
His theory does not simply explain the behaviors of galaxies; it predict them. The problem with theories is that they can explain just about anything. If you walk into a room and see that the lights are on, you can develop a theory that cosmic rays from the sun are hitting hidden mirrors in just the right way to light up the room. Another theory might be that someone flicked the light switch. One way to separate good theories from bad ones is to see which theory makes better predictions.
Recent analysis of Mond shows that it makes significantly better predictions than standard dark matter models. What that means is that, while dark matter can explain the behavior of galaxies quite well, it has little predictive power and is, at least on this front, an inferior theory.
Only more data and debate will be able to settle the score on dark matter and Mond. However, Mond coming to be accepted as the best explanation would shatter decades of scientific consensus and make one of the more mysterious features of the universe much more normal. A modified theory may not be as sexy as dark, unseen forces, but it may just have the advantage of being better science.