What do you see when you think of our universe?
Whatever it might be...
...that’s just the start of the story
The stuff of matter, the stuff that we’re made of...
...is not the whole story
As scientists have measured...
...the way that the galaxies move around...
...the way that the universe is actually expanding...
...they realised that they’re not seeing most of the universe
So what makes up everything else?
This could well be the biggest mystery of all
For a long time when people looked up into space...
...they thought that everything in the universe...
...stars, galaxies, planets, us were made of the same thing ...
...which is atoms and molecules that contain protons and neutrons...
...but as more data comes in, there are lots of anomalies
There’s actually much, much more other stuff out there
Normal matter, the stuff we can see or detect...
...makes up just 5% of the composition of the universe
Scientists think that the rest of the universe...
...is composed of two other things: dark matter and dark energy
So far, so good. But there’s a problem
Dark matter and dark energy are words for stuff that scientists don’t...
...really understand, they’re kind of like metaphors...
...for ignorance about the universe
All we know is that we can’t see either of them...
...and they’re very hard to detect
Neither dark matter nor dark energy absorb or emit light...
...making them completely invisible to modern scientific instruments
So the obvious question is how do scientists even know they exist?
Dark matter cannot be detected by traditional space...
...observatories or telescopes on the ground because all of those machines...
...use electromagnetic radiation—light, basically—at different frequencies...
...and dark matter doesn’t emit that kind of radiation
The only way that we know dark matter exists is by its gravitational effects
These effects were first observed in 1933 by the astronomer Fritz Zwicky
While studying a distant cluster of galaxies...
...he and his team noticed something very strange
They realised that the galaxies on the outer rim were moving way too fast...
...there wasn’t enough gravitational force within that galaxy cluster...
...to keep these galaxies at the edge
If these galaxies at the edge are moving that fast...
...they should have flown away, but they weren’t flying away
Zwicky and his team wrote that there must be something else present...
...some missing mass—holding the galaxies together
The overall name for this missing mass that we can’t see is dark matter ...
...and that missing mass was observed in lots of other places
The presence of dark matter seems to act like an invisible glue...
...holding the normal matter in the universe together
But finding out what it actually is...
...is like trying to solve a detective mystery
Any questions? All right, you all have your instructions , let’s go
You have to identify the likely suspects
First, there are MACHOs...
...Massive Astrophysical Compact Halo Objects
These are mostly dead or failed stars like neutron stars...
...which are leftover when a massive star runs out of fuel and collapses
Or brown dwarfs, which are enormous clumps of atoms...
...that never quite managed to develop into stars
These sound promising at first, but nowadays...
...many scientists think that MACHOs are unlikey to be behind dark matter
So maybe it’s WIMPs
But, Weakly Interacting Massive Particles are just one of the hypothetical particles...
...that scientists are actually looking for
There are a dizzying array of other potential suspects...
...many of which are also yet to be discovered in practice
Little wonder then, that many eyes turned...
...to the Large Hadron Collider at CERN in Geneva
Scientists there have been searching for possible dark matter particles...
...for more than a decade
But they’re yet to find anything...
...so now what?
As well as looking in particle colliders to make the particles of dark matter...
...another way is to look out into space and look for the gravitational effects of it
There’s a new class of instruments called gravitational-wave detection machines...
...which really only started working a few years ago on Earth
These can detect the ripples in space-time that gravitational objects create
If there’s huge clumps of dark matter somewhere doing something interesting...
...they will potentially emit gravitational waves...
...and that’s another way in to finding out what they might be
Hi I’m Caitlin and I directed this film...
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...and now back to the film
If you think dark matter is a challenging mystery to unravel...
...that’s nothing compared to dark energy
Dark energy is similar to dark matter in just a couple of ways
It’s invisible and it has the word dark in it
Beyond that, well, scientists are struggling...
...but they think it exists because of one incredible discovery
Ever since the Big Bang, the universe has been expanding
Scientists used to believe that the pull of gravity...
...would eventually cause its expansion to slow down
But in the late 1990s, they actually found the opposite
The expansion of the universe was speeding up
This finding was extraordinary
Imagine you threw a ball into the air...
...and instead of being pulled back down via gravity...
...it just continues to fly straight up into the sky
Well, the discovery of dark energy was kind of the same thing
It had scientists stumped
Now, this was a big surprise and couldn’t be explained...
...using the physics that astrophysicists already had at the time
So as a placeholder to try and explain why the universe is not only expanding...
...but that expansion is accelerating...
...they created this stuff called dark energy
Scientists think that dark energy accounts...
...for the majority of the energy in the universe...
...but most of its properties still remain a mystery
Dark energy really is beyond any theories of established physics right now
All we know about it is that it has the opposite effect of gravity
Gravity pulls things together and dark energy pushes things apart
Now, we don’t know if it’s a particle or a fluid or a force...
...that’s pushing things around everywhere...
...or whether it’s a property of space itself
One theory is that dark energy could be something like a fluid that fills up space
Some theorists have call it “quintessence”
Another possibility takes us back in time...
...to one of the fathers of modern physics
Albert Einstein realised that empty space isn’t simply nothing
He argued that there must be a force at play there...
...to counteract gravity, otherwise the universe would just collapse
To account for this force...
...he added a term called the cosmological constant to his theory of gravity
Einstein believed that the universe was neither expanding nor contracting
When it was later discovered that the universe is expanding...
...he discarded the idea of a cosmological constant
He later called it his biggest blunder
But what if it wasn’t?
The thing is, as we discover what dark energy is, it seems to quite neatly match...
...with the idea of a cosmological constant
So some people are trying to introduce the idea of a cosmological constant...
...back into general relativity...
...to try and explain some of the things that dark energy does...
...so Einstein thought he was wrong, but he might have been right
The cosmological constant or something like it...
...could be the force driving the expansion of the universe
Whichever theory ultimately explains dark energy...
...unravelling its mystery and working out how strong its force really is...
...may give us a glimpse into the future of our universe
Maybe dark energy will keep pushing galaxies away from each other...
...until they become lonely dots in the vast emptiness of space
The stars would stop shining, everything would go cold...
...and the universe will face what’s called the Big Freeze
Or perhaps we’re heading for the equally terrifying Big Rip...
...where dark energy overcomes gravity and rips the universe apart
If you want to know where the universe is going...
...you need to understand dark energy, because that’s the force ...
...or energy at the most cosmic scale that is shaping the structure of the universe
In the meantime, scientists will continue their efforts...
...to solve the mysteries of dark energy, dark matter...
...and the universe itself
Understanding these phenomena are really important...
...for scientists to get a grip on what the universe actually is now...
...to understand the Big Bang...
...but also what’s going to happen far off into the future
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