About 27% of the universe is made up
of a substance we can't see called dark matter.
Scientists are trying to figure out what dark matter is
but each possible answer has its own set of problems.
Here are five candidates for dark matter
that range from the unlikely to the probably
but we still haven't found it yet.
I'm Sophie, and welcome to the Countdown.
Some scientists aren't sure dark matter even exists.
After all, we can't see it directly.
What we see are galaxies and galaxy clusters
held together more tightly than we'd expect.
Researchers think this clinginess is due to the force
exerted by unseen matter.
But other scientists want to explain it another way.
By modifying the theory of gravity.
They can test these alternatives against observations
from instruments such as NASA's
Wide Field Infrared Survey Telescope.
And the British Square Kilometer Array.
But they haven't been able to explain the motion of all objects in space.
There must be some unseen element
and it's probably dark matter.
Maybe the universe's extra mass is coming from large objects
that are just difficult to observe.
They're called massive compact halo objects or MACHOS.
These might be black holes which emit no light
or they could be cold faint bodies like brown dwarfs,
failed stars without enough mass to perform fusion.
Or, neutron stars, which pack the sun's mass
into an object a quadrillion times smaller.
Literally, a quadrillion.
Unfortunately, physicists have calculated the amount of dark matter
in the universe and MACHOS can't account for it all.
With MACHOS off the table, dark matter must be made up of more exotic particles,
which don't normally interact with regular matter.
Take neutrinos for example.
Tens of thousands of these particles string through your body every second,
without having any impact.
Some think dark matter could be a particular type of neutrino,
called the sterile neutrino.
But sensitive detectors like Firmi Labs MiniBooNE
have been hunting for sterile neutrinos, and have yet to find any.
Another negative for sterile neutrinos
is the fact that they are so called hot particles,
moving at near light speed.
And hot dark matter fails to explain
how galaxies are distributed across the universe.
So dark matter consists of particles
that very rarely interact with regular matter,
and move significantly slower than the speed of light.
This is known as cold dark matter.
A particle called the axion could make up this kind of matter.
Unfortunately, it's still hypothetical.
We haven't directly observed any axions.
Yet.
The axion dark matter experiment, or ADMX,
at the University of Washington, is currently on the hunt.
But axions aren't quite as popular as WIMPS.
The number one dark matter candidate.
WIMPS stands for Weakly Interacting Massive Particle.
Like axions, they're a form of cold dark matter.
But they're bigger, and more likely to interact with regular particles.
You might think this would make WIMPS
easier to detect, but we haven't found anything definitive.
The cryogenic dark matter search at the Fermilab
has seen promising WIMP hints,
but the large underground xenon experiment, or LUX,
which is the most sensitive detector yet,
failed to reproduce these sightings.
Still, this doens't mean WIMPS are out for the count.
It just means we might be looking in the wrong place.
The experiments I mentioned were only studying a limited range of energies,
so we can still hold out for WIMPS.
Because if they're off the table, then we're really in the dark.
I'm Sophie Bushwick, and that's your Coundown.
For more spacey stuff, visit the Spacelab Channel on Youtube,
or follow us on twitter, @sa_spacelab.
And if you've got any topics you'd like to see in the future,
let us know in the comments.
(off camera) Hey Sophie, would you rather fight
a hundred WIMPsized MACHOs,
or one MACHO sized WIMP?
I'd rather fight a hundred WIMP sized MACHOS
because the size of a WIMP is so infinitesimal
that they couldn't do anything to me.