Dark Matter 101: Looking for the missing mass

Here’s the deal — here at NASA we share all

kinds of amazing images of planets,

stars,

galaxies, astronauts,

other humans,

and such, but those photos can only capture part of what’s out there. Every

image only shows ordinary matter (scientists sometimes call it baryonic

matter), which is stuff made from protons, neutrons and electrons. The problem

astronomers have is that most of the

matter in the universe is not ordinary matter – it’s a mysterious substance called dark matter.  

What

is dark matter? We don’t really know.

That’s not to say we don’t know anything about it – we can see its effects on

ordinary matter. We’ve been getting clues about what it is and what it is not

for decades. However, it’s hard to pinpoint its exact nature when it doesn’t

emit light our telescopes can see. 

Misbehaving

galaxies

The first hint that we might be missing

something came in the 1930s when astronomers noticed that the visible matter in

some clusters of galaxies wasn’t enough to hold the cluster together. The

galaxies were moving so fast that they should have gone zinging out of the

cluster before too long (astronomically speaking), leaving no cluster behind.

Simulation credit: ESO/L. Calçada

It turns out, there’s a similar problem with individual galaxies.

In the 1960s and 70s, astronomers mapped out how fast the stars in a galaxy

were moving relative to its center. The outer parts of every single spiral

galaxy the scientists looked at were traveling so fast that they should have

been flying apart.

Something was missing – a lot of it! In

order to explain how galaxies moved in clusters and stars moved in individual

galaxies, they needed more matter than scientists could see. And not just a little more matter. A lot … a lot, a lot. Astronomers

call this missing mass “dark matter” — “dark” because we don’t know

what it is. There would need to be five times as much dark matter as ordinary

matter to solve the problem.  

Holding

things together

Dark matter keeps galaxies and galaxy clusters

from coming apart at the seams, which means dark matter experiences gravity

the same way we do.

In addition to holding things together, it

distorts space like any other mass. Sometimes we see distant

galaxies whose light has been bent around massive objects on its way

to us. This makes the galaxies appear stretched out or contorted. These distortions provide another measurement of dark

matter.

Undiscovered

particles?

There have been a number of theories over the

past several decades about what dark matter could be; for example, could dark

matter be black holes and neutron stars – dead stars that aren’t shining anymore?

However, most of the theories have been disproven. Currently, a leading class

of candidates involves an as-yet-undiscovered type of elementary particle

called WIMPs, or Weakly Interacting Massive Particles.

Theorists have envisioned a range of WIMP

types and what happens when they collide with each other. Two possibilities are

that the WIMPS could mutually annihilate, or they could produce an

intermediate, quickly decaying particle. In both cases, the collision would end

with the production of gamma rays — the most energetic form of light — within the detection range of our Fermi Gamma-ray Space Telescope.

Tantalizing

evidence close to home

A few years ago, researchers took a look at

Fermi data from near the center of our galaxy and subtracted out the gamma rays

produced by known sources. There was a left-over gamma-ray signal, which could be consistent with some forms of dark matter.

While it was an exciting finding, the case is

not yet closed because lots of things at the center of the galaxy make gamma

rays. It’s going to take multiple sightings using other experiments and looking

at other astronomical objects to know

for sure if this excess is from dark matter.

In the meantime, Fermi will continue the search, as it has over its 10 years

in space. Learn

more about Fermi and how we’ve been celebrating its first decade in space.

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