Dark Matter: What Role Does it Play in Galaxies?

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

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.

Every massive galaxy has a black hole at its center, and the heftier the galaxy, the bigger its black hole. But why 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.  

To investigate the link between dark matter halos and supermassive black holes, astronomers Akos Bogdan and his colleague Andy Goulding (Princeton University) studied more than 3,000 elliptical galaxies. These are roughly egg-shaped collections of stars with black holes at their hearts.

They 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, because the more dark matter a galaxy has, the more hot gas it can hold onto.

They found a distinct relationship between the mass of the dark matter halo and the black hole mass, in a relationship stronger than that between a black hole and the galaxy's stars alone.

This connection is likely to be related to how elliptical galaxies grow. An elliptical galaxy is formed when smaller galaxies merge, their stars and dark matter mingling and mixing together. 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.

Astronomers strongly suspect that dark matter affects the growth of other types of galaxies, too, and may well have an effect on stars and planets within our galaxy. 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 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.


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.