Dark Matter: What Role Does it Play?

This illustration shows two spiral galaxies - each with supermassive black holes at their center - as they are about to collide and form an elliptical galaxy. New research shows that galaxies' dark matter halos influence these mergers and the resulting growth of supermassive black holes. NASA/CXC/M.Weiss

We've all heard about dark matter — that mysterious "stuff" of the cosmos that so far hasn't been detected directly but can be inferred by its gravitational effect on "normal" (what scientists call "baryonic") matter. Astronomers know it's important and it plays a role, but what is that role? 

In our universe, dark matter outweighs normal matter — the everyday stuff we see all around us by a factor of 6 to 1. The gravitational effect of all that matter holds together galaxies and galaxy clusters. Every galaxy is surrounded by a halo of dark matter that weighs as much as a trillion suns and extends for hundreds of thousands of light-years.

The biggest question about dark matter is: what's it made of?  Is it the so-called "hot" dark matter? Or "cold" dark matter? Astronomers are still trying to figure that out. Dark matter can't be seen, felt, tasted, smelled, or imaged. It can, however, be detected by its influence on other material in the universe. That includes its gravitational pull. But, there are other aspects of dark matter than researchers are still discovering and explaining. Once they get a good idea of what it actually is, that is, what particles it's made of, they'll be able to characterize it more fully.  It's only a matter of time before that happens. 

Galaxies, Dark Matter, and Black Holes

Astronomers know certain things about galaxies: they have stars, planets, nebulae, black holes, and a lot of dark matter. ​Each massive galaxy has a black hole at its center. The heftier the galaxy, the bigger is its black hole. But how are the two related? After all, the black hole is millions of times smaller and less massive than its home galaxy. Astronomers study football-shaped collections of stars called elliptical galaxies to understand the connection between a galaxy and its black hole. It turns out that the invisible hand of dark matter somehow influences black hole growth and the formation of galaxies.  ​The existence of this matter that we can't see or touch was first postulated by astronomer Fritz Zwicky in the early 20th century, and later observed and verified by a team of observers led by astronomer Vera Rubin

Elliptical galaxies are roughly egg-shaped collections of stars with black holes at their hearts. Scientists used star motions as a way to weigh the galaxies' central black holes. X-ray measurements of hot gas surrounding the galaxies helped weigh the dark matter halo. It turns out that the more dark matter a galaxy has, the more hot gas it can hold onto. So, in a galaxy with a large dark matter "halo" surrounding it, the relationship between the two is stronger than that between a black hole and the galaxy's stars.

This connection is probably related to how elliptical galaxies grow. They form when smaller galaxies merge, and the stars and dark matter mingle and mix. Because the dark matter outweighs everything else, it molds the newly formed elliptical galaxy and guides the growth of the central black hole. The merger creates a gravitational blueprint that the galaxy, the stars, and the black hole will follow in order to build themselves. 

Dark Matter and Other Galaxies

Astronomers strongly suspect that dark matter affects the growth of other types of galaxies, too. Recent theoretical studies of dark matter and its influence on objects in the galaxy indicate that Earth itself, and perhaps even the life it supports, have been affected by the dark matter in some way as our Sun and planets traveled through the galaxy over hundreds of millions of years.

The galactic disk—the region of the Milky Way Galaxy where our solar system lives — is crowded with stars and clouds of gas and dust, and also a concentration of elusive dark matter — small subatomic particles that can be detected only by their gravitational effects.  As Earth (and presumably planetary systems around other stars) travel through the disk, dark matter accumulations disturb the orbits of far-flung comets, sending them on collision courses with planets. 

Dark Matter and Our Planet

It also seems that dark matter can apparently accumulate within Earth's core. Eventually, the dark matter particles annihilate each other, producing considerable heat. The heat created by the annihilation of dark matter in Earth's core could trigger events such as volcanic eruptions, mountain building, magnetic field reversals, and changes in sea level, which also show peaks every 30 million years. 

Dark matter, it seems, has a lot to answer for in the universe. It's an amazingly effective material, even though it hasn't yet been seen. Its invisible hand is felt everywhere.