Observable, ordinary matter makes up just
5% of the universe.
Everything we can see consists of it; all
the stars in the sky, all of the planets,
every single human being and all the food
you’ve ever eaten.
But will we ever be able to observe everything
else that’s out there?
This is Unveiled, and today we’re answering
the extraordinary question; what if we could
see dark matter?
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Around 25% of everything in the universe is
dark matter, an unknown type of matter with
mysterious properties we don’t understand.
The other 70% of the universe (that isn’t
dark or ordinary matter) is made up of dark
energy, which is in many ways even more elusive
than dark matter is.
The word “dark” in this context simply
means they’re both unknown rather than that
they’re connected to each other, so we can
certainly talk about them in isolation.
Dark energy is the substance we think causes
cosmic expansion.
Dark matter, on the other hand, is speculated
to exist because of how we currently understand
gravity to work; it’s believed to be there
because of various gravitational effects we’ve
observed which don’t make sense without
it.
Overall, while we know roughly what dark matter
does and where it probably is, we’re still
not sure on exactly what it is.
The gravitational effects we’re talking
about are seen most clearly in the formation
of galaxies.
Galaxies are enormous, and we initially expect
that things further out from the galactic
centre should orbit at slower and slower speeds.
But this has been found not to be the case;
distant objects orbit at roughly the same
speeds as close ones; at around 250 kilometres
per second in the Andromeda galaxy, for example.
It was this which gave rise to the theory
of dark matter, with the suggestion that it’s
this strange substance that helps galaxies
to keep their otherwise inexplicable shape.
Handily, we can also plot the placement of
dark matter through gravitational lensing,
a phenomenon where light from distant galaxies
is distorted by the gravitational pull of
massive objects in its path.
In some cases, although we can observe light
being warped, we aren’t able to detect the
object responsible - so dark matter is to
blame.
It’s either that, or almost everything else
we know about gravity - starting with Einstein’s
theory of relativity - is wrong.
Today, the existence of dark matter is viewed
as more likely.
But all of our observations, suppositions
and theories still aren’t enough to prove
that dark matter definitely exists.
There are, though, many experiments running
around the world trying to do just that - by
directly detecting it.
SNOLAB is an operation deep inside a nickel
mine in Ontario, Canada, which has a handful
of dark matter detectors within.
One detector, known as DAMIC, is trying to
take images of various particles to hopefully
one day find a new one that could be deemed
“the first dark matter particle”.
In the same facility, DEAP-3600 is trying
to detect dark matter particles as they pass
through argon.
Elsewhere, other experiments include XENON1T
in Italy, which is trying to discover “WIMPs”
– or, “weakly interacting massive particles”
– by observing whether xenon gas will light
up when one passes through it.
As of late 2019, however, neither these nor
any other detectors around the world have
been successful in their search, though physicists
stress that this isn’t necessarily a bad
thing.
We still don’t know what dark matter particles
look like or how they behave, but all of these
duds at least show us what they don’t look
like and how they don’t behave.
By process of elimination and by tweaking
the experiments we’ve set up so far, we
may one day be able to “see” the particles
we so ardently suspect are there.
To take a different approach, we’ve also
tried to map dark matter through simulations
and artist renditions of outer space.
These are again largely based on the perceived
behaviour of gravity, especially in the formation
of galaxies, but some sims have led some scientists
to think that the Milky Way and most other
galaxies might have a “dark matter halo”.
If true, it’s this halo that keeps a galaxy
so rigidly structured, with the dark matter
inside the halo gravitationally bound together
just like normal matter is.
The Milky Way’s proposed halo is thought
to be spherical and to comfortably encompass
our entire galaxy - being much, much larger
than the Milky Way itself.
Interestingly, however, according to a November
2019 study published in “Nature Astronomy”,
there are some dwarf galaxies which don’t
appear to have this halo, with at least nineteen
structures out there seeming not to harbour
dark matter in the same way - they behave
as if all the mass they have is the regular
matter we can see.
These seeming anomalies serve to throw the
debate wide open, though, with some arguing
that the galaxies without simply “lost”
their dark matter somehow over time… while
others see the absence of dark matter in these
galaxies as evidence that dark matter doesn’t
exist at all.
Currently, we’ve no way to tell which side
is right, but the people who do believe in
dark matter are still in the majority.
Since we can’t see any subatomic particles
with the naked eye, were those that make up
dark matter to be something that we understood
more, then despite the fact that we’d be
surrounded by it all the time… we still
might not be able to “see” a lot of it
for ourselves.
We can’t see the individual particles in
the air, for example, but we know that they’re
there because of our ability to breathe.
This means that the predicted dark matter
halo, where the concentration of dark matter
is thought to be especially high, would be
the most visible source of dark matter for
ordinary people.
We can already see certain sprawling, dazzling
parts of the Milky Way in the night sky, so
if the halo were suddenly visible it would
simply become another of its most prominent
features.
But would dark matter still be dark matter
in this scenario?
Since the reason we’re so interested in
it is because it’s completely unknown and
strange, if we could see it, we’d be able
to study and understand its properties with
far more ease.
If we could look into the sky and see the
dark matter halo, we would understand with
close to certainty that that halo is how the
galaxy keeps its shape.
Or, if we could see dark matter but the halos
still didn’t exist, we’d know that it
really was our understanding of gravity that
would need another going over.
In either case, dark matter that’s visible
would need redefining; the mystery would be
lifted, and the “darkness” would have
disappeared.
And yet, the hypothetical unveiling of dark
matter to the world could bring with it countless
other problems to solve as well.
Some, less mainstream theories suggest that
if there’s dark matter there, then it could
potentially make up whole structures, perhaps
planets, maybe even lifeforms all of its own.
In this way, there could be an entire “shadow
universe” hidden in plain sight; beyond
our comprehension at the moment, but perhaps
not if we ever were to crack dark matter.
It sounds farfetched, but it might not be
as ridiculous as it seems.
Almost all observable matter is already one
specific type; baryonic matter, consisting
of baryons - usually protons and neutrons.
But non-baryonic matter definitely does exist…
with electrons, for example, technically being
non-baryonic particles called leptons.
We’ve already discovered other types of
lepton particle in the past, like taus and
muons, but also neutrinos, which were once
as elusive as dark matter is today.
It took more than twenty-five years after
neutrinos were first predicted for them to
actually be discovered, so could our understanding
of dark matter be following a similar path?
Given how little we currently know about it,
perhaps it isn’t hard to imagine that “dark
matter” could contain all kinds of things…
So, it arguably could populate a whole periodic
table all of its own, accounting for entire,
secret civilizations that we might someday
be able to communicate with.
It’s clearly a long way between now and
then, but visible dark matter would see one
of the greatest questions in modern physics
finally answered.
We would have an unprecedented understanding
of how the universe and everything - truly
everything - in it forms and functions.
And that’s what would happen if we could
see dark matter.
What do you think?
Is there anything we missed?
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