What Is Dark Matter?
Astronomers today generally believe in the existence of dark matter. What is dark matter? If I knew the answer to that question, I’d probably get a Nobel Prize in Physics. Yes, it’s that big of a problem in modern astronomy and physics. Before we can answer this question a little better, we briefly need to review the history of our understanding of dark matter.
Lighted Mass Versus Dynamic Mass
The evidence for dark matter goes back more than 80 years. Stars congregate into large structures called galaxies. The sun is in the Milky Way Galaxy, a collection of a couple hundred billion stars. Astronomers long ago realized that large collections of stars, such as galaxies, follow a mass-luminosity (M/L) ratio. This makes sense, because more mass usually translates into more stars, which results in more light. If we measure how bright a galaxy is, then multiplying the light by the M/L ratio reveals the galaxy’s lighted mass.
But there is a more direct way to measure galaxy mass. The acceleration of gravity depends upon mass, so measuring the force of gravity reveals mass. We do this when we weigh ourselves. The process is different for weighing astronomical bodies, but the physics is the same. Since orbital motion results from gravity, we can determine mass by measuring orbital motion. We use the moon’s orbit around the earth to measure the earth’s mass. Similarly, we study the orbital motions of the natural satellites, or moons, of other planets to measure the masses of the planets. We measure the orbital motion of the planets to determine the sun’s mass. God made many stars in binary systems, two stars orbiting a common center of mass, from which we can determine the masses of many stars. Mass determined from orbital motion is dynamic mass.
Galaxies tend to exist in large clusters. Presumably, the galaxies in a cluster are orbiting around a common center of mass. In the 1930s, an astronomer measured the motions of galaxies within clusters and found that the dynamic masses were always larger than the lighted masses of the clusters. A few years later, two other astronomers measured the orbital motion of objects within the Andromeda Galaxy, the closest galaxy of any size, to determine its dynamic mass. They found that the dynamic mass of the Andromeda Galaxy was much greater than its lighted mass too.
Dark Matter Versus MOND
Astronomers largely ignored this “missing mass” problem for a half-century. However, a series of papers in the 1970s studying orbital motion within many galaxies showed the same result—the dynamic mass was always far greater than the lighted mass, typically by a factor of 10. By the 1980s, astronomers stopped ignoring this problem as they came to grips with the fact that most of the universe’s mass was invisible. What was this dark matter, as it was soon called? It was very clear that it wasn’t “normal matter,” like the stuff that we are made of. Different theories soon arose about what exotic particles might account for dark matter. Tests of various theories were devised, and the results eliminated the theories. Conclusion: most of the universe’s mass is probably in a form of matter that we haven’t yet contemplated.
Most of the universe’s mass is probably in a form of matter that we haven’t yet contemplated.
Some physicists found this prospect unsettling, so they opted for an alternate solution: modifying the basic physics of gravity. MOND (for MOdified Newtonian Dynamics) makes a small change in the way that gravity works. Over small distances (less than thousands of light years), the difference between MOND and traditional Newtonian gravity is not noticeable. But on a galaxy-sized scale, the difference shows up. Physicists fit the orbital data from within galaxies to develop their model of MOND. However, the majority opinion, particularly among astronomers, is that dark matter is the correct resolution to this question.
What Was Recently Discovered?
A recent discovery revealed that a nearby galaxy, NGC 1052-DF2, has little or no dark matter.1 NGC 1052-DF2 is a little more than 60 million light years away and is about the same size of the Milky Way. However, NGC 1052-DF2 is much fainter than the Milky Way or other galaxies of similar size. Application of the M/L ratio reveals a lighted mass of about 200 million times that of the sun. The orbital motion of 10 objects, presumed to be globular clusters, produced a dynamic mass only slightly larger than the lighted mass. Considering the likely errors involved, this result is consistent with little or no dark matter in this galaxy. However, this galaxy certainly is large enough for MOND to operate, if MOND is correct. The fact that the dynamic mass of this galaxy is consistent with its lighted mass would seem to eliminate MOND as a viable theory. Paradoxically, the discovery that there is no dark matter in this particular galaxy amounts to evidence that dark matter exists in other galaxies.
What Does This Mean to Us?
As I previously stated, the question of dark matter is debated among physicists and astronomers, but dark matter is the dominant theory. However, among creationists, there is much more skepticism about dark matter. Why is this? This may stem from dark matter’s invocation in cosmology today. Once astronomers became convinced of the reality of dark matter three decades ago, cosmologists realized that dark matter was another free parameter that they could manipulate in the big bang model. Consequently, some critics of the big bang have viewed it as a rescuing device. However, long before dark matter was used to prop up the big bang, there was good observational evidence for dark matter. We ought not to be confused by appeals to dark matter in the context of historical science.
While dark matter may not turn out to be the correct solution, it is the best bet so far. Isn’t it humbling that God created the universe so complex that we don’t even know what most of it is made of?