It appears gas and dust aren’t the only things
existing between the stars. There isn’t enough
visible material to account for the gravity needed to
hold the rapidly rotating Milky Way together, so it
has been suggested there’s some invisible material
called dark matter that lurks all around us – including
in interstellar space.
Astronomers initially thought that dark matter
might just be ordinary stuff that was too dim to see- objects such as brown dwarfs, stray planets and
solar-mass black holes. These came to be known as
MACHOs (Massive Compact Halo Objects).
However, they should be detectable by other
means, and we haven’t found enough of them yet.
Now, the leading modern idea is that dark matter
is made of WIMPs (Weakly Interacting Massive
Particles) – a new form of matter that doesn’t fit with
the Standard Model of particle physics. But so far,
snaring a WIMP, or even proving their existence, has
been out of reach. However, according to the
University of Lancashire’s Cristina Popescu,
studying the ISM might well help.
The Milky Way is full of cosmic rays – high energy
particles created by apocalyptic events such as
supernova explosions. When these rays strike
interstellar dust and gas, they create a flood of
gamma rays. Yet there are more gamma rays in the
Milky Way than astronomers expected, particularly
near the centre of the Galaxy.
“You look at the spectrum of what’s left over, and
it matches what you’d expect from the annihilation
of WIMPs,” says Popescu. Annihilation is where two
WIMPs collide to create gamma rays. “Of course,
this all relies on a good understanding of the
interstellar medium,” she says.
Unfortunately, the Voyager probes were launched
before the idea of WIMPs was dreamt up. However,
annihilating WIMPs should produce particles that
they could detect, at least in theory.
Some accounts of dark matter also invoke the
presence of a so-called dark photon. Proposed in
2008, this hypothetical particle is supposed to carry
the force of dark matter, much like an ordinary
photon carries the electromagnetic force. In 2015, a
team of French physicists at the Université Grenoble
Alpes used data from the Voyager probes to look for
the magnetic signature of dark photons in the ISM.
While they didn’t find it, they did put some
constraints on its properties, so the net is narrowing.
Then, last year, a team of astronomers from
Australia and the UK looked at the effect on galaxies
of ultra-compact mini-halos of dark matter
(UCMHs). These are particularly dense dark matter
clumps. They calculated that if only 1 per cent of a
galaxy’s dark matter was in the form of UCMHs, the
heat from dark matter annihilations would be
enough to eject all of the gas out of the interstellar
medium in the immediate vicinity. This would
prevent stars forming within around 3,200 light-
years of the mini-halo. So looking for such gaps in
the ISM is another way to learn more about the
distribution of dark matter. ßColin Stuart is an astronomy writer and co- author of The Geek
Guide To Life. He tweets from @skypondererDARK MATTER
What can venturing beyond the Solar
System tell us about this most elusive
cosmic phenomenon?
PHOTO: NASA, SCIENCE PHOTO LIBRARY