Shedding some light on Dark Matter

What is Dark Matter? Why did scientists propose Dark Matter in the first place?

For over 80 years, astronomers have had a problem. They can measure the amount of matter we can see, the luminous matter of the Universe, consisting of dust, gas, and stars. Stars emit light, and that light illuminates the gas and dust in the interstellar medium, and in some places causes the gas to glow with its own light. But what if stars aren’t illuminating some of the matter? We wouldn’t see it. But even if matter isn’t emitting or reflecting light, we can tell where it is by the gravitational effects it produces. And back in the 1930s, astronomers discovered that the orbits of stars within our galaxy indicated a lot of mass, much more than could be accounted for by the luminous matter. Careful observations of clusters of galaxies showed a similar pattern. The orbits of galaxies in large clusters indicated a lot more mass than could be seen. In fact, when astronomers started looking, it seemed as if everywhere they looked, if the scale was galaxy sized or larger, they saw indications of much more mass that could be accounted for by the luminous matter. Galactic rotations, the distribution of gas and dust, gravitational lensing, and recently the tiny asymmetries in the Cosmic Microwave Background; all gravitational calculations showed much more mass than we could see.

What form does the missing mass take? If the mass consists of baryons (that is, just normal matter made of protons, neutrons, and electrons), there are two problems. One, there are theoretical reasons for believing the Big Bang simply didn’t produce as much baryonic matter as we need to account for the gravitational effects. Two, since baryonic matter interacts with light, we’d see it if it were near stars. One solution to the latter problem is to put the dark baryonic matter out in the halo of galaxies where there are few stars and therefore little light. But even there, we should see some interaction with the dim light from distant objects, and astronomers have looked carefully without finding enough of those interactions to account for the missing mass if that mass consists of baryons. So the missing mass is unlikely to be baryonic matter.

Our missing mass must therefore consist of exotic particles. The particles cannot be baryons, they cannot interact with the electromagnetic field (in other words, they don’t emit or reflect light), and they must have mass.  There are various candidate particles; axions, supersymmetric particles, and neutrinos. There is some observational evidence that neutrinos can’t be a significant fraction of the missing mass. But as of today, we simply do not know what kinds of particles make up the bulk of the Dark Matter of the universe.

There is at least one other explanation for the missing mass. If our current understanding of gravity is slightly wrong at very large distances, we can account for observed large-scale orbital dynamics with just luminous matter. This theory, or really theories, since there are several versions of gravity modification, was first proposed in the early 1980s. However, if the luminous matter produces the bulk of the gravitational effects, the center of gravity calculated using luminous matter in a system also must be the center of gravity calculated from the orbital dynamics of the system. Observations of at least one set of colliding galaxies show the center of mass of the luminous matter is not the same as the center of mass calculated from orbital dynamics. It turns out to be not so easy to adjust the non-Newtonian gravity theories to account for these observations. So for now, most cosmologists and astronomers side with the Dark Matter explanation (exotic particles with mass and no electromagnetic interaction) to solve the missing mass problem.

And that’s what Dark Matter is and why scientists think it exists, even though no one has ever detected Dark Matter itself, aside from its gravitational effects.

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