Dark Matter: Is That What’s at the Centre of the Milky Way?

Dark Matter has been detected at the centre of the Milky Way galaxy. Or has it? Catch up on the debate and find out why it matters in the story of the universe.

The Chinese philosopher Lao Tzu famously wrote in the Tao Te Ching, “Thirty spokes connect to the wheel’s hub; yet it is the center hole that is of benefit. Clay is shaped into a vessel; yet it is the emptiness it surrounds that is of benefit. Walls are put up for a house yet it is the space within that is of benefit. We labour for what is there, but we benefit from what is not.”

Sometimes it’s what we don’t see that matters most.  That’s how it is with what cosmologists call dark matter.  It’s not that it isn’t there, but in a way, it’s missing.  It all started when astronomer Fritz Zwicky was studying galaxies in the Coma Cluster in 1933.

When Zwicky compared the total mass of the galaxies he was looking at with the gravitational effects he saw, they didn’t match.  In fact, there was 500 times more gravity than Zwicky could account for based on the number of stars in a typical galaxy.  He wrote, “”dark matter is present in a much greater amount than luminous matter.”  And that’s the issue.

Dark matter makes up 27% of the universe

Later discoveries confirmed Zwicky’s conclusions about this “missing matter.”  Today, we know that Zwicky’s dark matter makes up about 27% of the known universe.  The matter that we can actually see and study is only about 5% of what exists.

Scientists don’t know what dark matter is, but they have a pretty good idea of what it is not.  It’s not stars or planets because they emit or reflect light and dark matter doesn’t.  It’s not dark clouds of the subatomic particles physicists call baryons, because it doesn’t absorb radiation the way baryon clouds do. 

We can also rule out anti-matter because dark matter doesn’t emit the same kinds of gamma rays that anti-matter does when it annihilates with matter.  Lastly, it also isn’t black holes because it doesn’t bend the light passing by the way a black hole does.

Dark matter consists of some sort of particle

The working hypothesis that most cosmologists support is that dark matter consists of some sort of particle.  After all, the matter we know about consists of elementary particles that form into atoms.  One possibility is a hypothetical particle theorists call the axion.  Axions behave in ways that might explain the rotation patterns in the “halo” of dark matter scientists believe surrounds galaxies. 

The other hypothetical particle researchers are looking for is the Weakly Interacting Massive Particle (WIMP).  The idea here is that whatever dark matter is, it interacts very weakly, if at all, with light.  As well, since it causes lots of gravity, it must have a substantial amount of mass.

The trouble is, nobody has ever seen or detected anything resembling a WIMP.  There are strong theoretical and mathematical arguments for their existence. Still, so far, they never seem to turn up in experiments meant to find them.

Gamma rays in Milky Way from dark matter

One of the more significant controversies surrounding dark matter came back into the news last week.  Back in 2009, Dan Hooper and Lisa Goodenough at Fermilab detected a glow of gamma rays coming from the centre of the Milky Way.  They called it the “galactic centre excess,” and they proposed that it could be from dark matter in the journal High Energy Physics.

The idea is that when two hypothetical particles of dark matter collide, they should destroy each other.  This would release high energy photons in the form of gamma rays.  Not everyone agrees that dark matter is causing the gamma rays in the galactic centre excess.

Particle astrophysicists, led by Tracy Slayter and her colleagues at MIT, claimed in the journal Physical Review Letters in 2015 that the galactic centre excess comes from a type of neutron star called a pulsar.  They maintained that a cluster of pulsars that were too faint for the Fermi Gamma-ray Space Telescope to resolve would produce the same result. 

Pulsars would produce the same result. 

They thought this was a more straightforward and more complete explanation.  Slayter and her research team pointed to the size and graininess in the images from Fermi, arguing that this meant they came from pulsars.

Rene Levesque once declared that “only an idiot never changes his mind.” To their credit, Tracy Slayter, along with her postdoctoral researcher Rebecca Leane have now announced in a new paper for Physical Review Letters that they may have spoken too soon. The pair took another look at the templates they used in their model to rule out other gamma-ray sources.

They found that these templates may have inadvertently taken out the dark matter signal they should have been looking for.  It seems that the templates may have thrown the baby out with the bathwater.  If so, Hooper and Goodenough may have been right all along.

May have taken out the dark matter signal

Even so, there are other skeptics.  Theorist Glennys Farrar at NYU told Science magazine, “I don’t see anything so far that can’t be explained with [conventional] astrophysics.”  Others are concerned that the fascination with dark matter may be overshadowing the debate.  They feel that scientists should be openly considering all possible explanations instead of focusing so heavily on the issue of dark matter.

Mattio DiMauro at the Goddard Space Flight Center expressed pleasure that the controversy has revived interest in getting to the bottom of what goes on at the centre of the Milky Way.  But, he told Science, “ I’m scared that it will revive only the interest in dark matter.”

Humanity needs to get a grip on dark matter

Still, humanity needs to get a grip on what dark matter is.  If nothing else, between dark matter and dark energy, it’s frustrating to not understand the nature of 96% of the known universe.  We’re curious creatures by nature and this state of affairs if very unsatisfying.

More importantly, to tell us the fate of the universe, cosmologists need to know its density.  They can’t find that out without an understanding of dark matter and its constituents.  Every culture needs to be able to tell the story of the universe.  Nobody likes a story where the storyteller doesn’t know the ending.

We always have more to learn if we dare to know.
Learn more:
Science (AAAS)
Possible Evidence For Dark Matter Annihilation In The Inner Milky Way From The Fermi Gamma Ray Space Telescope
Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy

Dark Matter Strikes Back at the Galactic Center
The 5 Big Questions We Need Cosmology to Answer
Ancient Galaxies and Even Older Dark Matter
Are Diamonds Cosmology’s New Best Friend?


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