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Dark Matter 101: Looking for the missing mass

Dark Matter 101: Looking for the missing mass

Here’s the deal — here at NASA we share all
kinds of amazing images of planets,
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.  


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. 


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!

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.  

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


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.

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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