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Matter, as we know it;

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Atoms, stars and galaxies, planets and 
trees, rocks and us.

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This matter accounts for less than
5% of the known universe.

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00:00:12,483 --> 00:00:18,062
About 25% is dark matter; 
and 70% is dark energy.

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Both of which are invisible.

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This is kind of strange because it
suggests that everything we experience is

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really only a tiny fraction of reality.

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But it gets worse.

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We really have no clue what dark matter
and energy are, or how they work.

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We are pretty sure they exist though.

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So, what do we know?

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Dark matter is the stuff that makes 
it possible for galaxies to exist.

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When we calculated why the universe
is structured the way it is,

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00:00:54,347 --> 00:00:57,927
it quickly became clear that there's just
not enough normal matter.

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The gravity of the visible matter
is not strong enough to form

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galaxies and complex structures.

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The stars would more likely be scattered
all over the place and not form galaxies.

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So, we know there is something else inside
and around them.

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Something that doesn't
emit or reflect light.

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Something dark...

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but besides being able to calculate
the existence of dark matter,

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we can see it... kind of.

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Places with a high concentration of dark
matter bend light passing near by.

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So we know there's something there that
interacts with gravity.

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Right now, we have more ideas about
what dark energy is not, than what it is!

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We know dark matter is not just clouds of
normal matter without stars,

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because it would emit particles
we could detect.

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Dark matter is not anti-matter,

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because anti-matter produces unique gamma
rays when it reacts with normal matter.

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Dark matter is also not
made up of black holes,

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very compact objects that violently affect
their surroundings,

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while dark matter seems to be
scattered all over the place.

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Basically, we only know 3 things for sure;

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1. Something is out there.

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2. It interacts with gravity.

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3. There is a lot of it.

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Dark matter is probably made up of a 
complicated exotic particle

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that doesn't interact with 
light and matter in a way we expect,

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00:02:23,741 --> 00:02:25,755
but right now, we just don't know.

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Dark energy is even more
strange and mysterious.

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We can't detect it, we can't measure it
and we can't taste it.

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But we do see its effects very clearly.

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In 1929, Edward Hubble examined how 
the wavelength of light

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emitted by distant galaxies, shifts
towards the red end

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of the electromagnetic spectrum
as it travels through the space.

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He found that fainter, more distant galaxies
showed a large degree of red shift;

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closer galaxies, not so much.

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Hubble determined that this was because
the universe itself is expanding.

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The red shift occurs,
because the wavelengths of light

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are stretched as the universe expands.

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More recent discoveries have shown that, 
the expansion of the universe is accelerating.

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Before that, it was thougth that the pull
of gravity would cause the expansion to

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either slow down or even retract and 
collapse in on itself at some point.

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Space doesn't change its properties
as it expands; there's just more of it.

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New space is constantly
created everywhere,

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galaxies are tight bound clusters of stuff
held together by gravity

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so we don't experience this expansion
in our daily lives.

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But we see it everywhere around us.

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Wherever there is empty space in the 
universe, more is forming every second.

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So, dark energy seems to be some
kind of energy intrinsic to empty space.

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Energy that is stronger than
anything else we know

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and it keeps getting stronger
as time passes by.

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Empty space has more energy than
everything else in the universe combined.

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00:04:02,072 --> 00:04:06,012
We have multiple ideas about 
what dark energy might be.

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00:04:06,124 --> 00:04:11,469
One idea is, the dark energy is not
a thing, but just a property of space.

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Empty space is not nothing, it has its own
energy.

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00:04:15,382 --> 00:04:18,451
It can generate more space and
is quite active.

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00:04:18,531 --> 00:04:22,829
So, as the universe expands,
it could be that just more and more space

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00:04:22,829 --> 00:04:27,975
appears to fill the gaps and this leads
to a faster expanding universe.

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This idea is close to an idea that 
Einstein had back in 1917,

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of the concept of a cosmological constant,

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a force that counteracted
the force of gravity.

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The only problem is, that when we tried
to calculate the amount of this energy

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the result was so wrong and weird,
that it only added to the confusion.

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Another idea is, that empty space is
actually full of

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00:04:50,418 --> 00:04:54,428
temporary, virtual particles that 
spontaneously and continually

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form from nothing and then dissapear into
nothing again.

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The energy form those particles
could be dark energy.

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00:05:01,907 --> 00:05:06,572
Or maybe dark energy is an unknown kind of
dynamic energy fluid or field

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which permeates the entire universe,
but somehow has the opposite effect

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00:05:11,122 --> 00:05:13,771
on the universe than normal
energy and matter.

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00:05:14,015 --> 00:05:18,591
But if it exists, we don't know
how and where or how we could detect it.

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00:05:18,742 --> 00:05:22,214
So there are still a lot of questions to
answer.

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00:05:22,321 --> 00:05:26,036
Our theories about dark matter and
dark energy are still just that;

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theories.

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On one hand, this is kind of frustrating,
on the other hand this is frontier science

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making it very exciting.

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It shows us that no matter how much
we feel we're on top of things,

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we are still very much
apes with smartphones, on a tiny

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fragile island in space looking into
the sky wondering how our universe works

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There is so much left to learn,
and that is awesome.

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This video is supported by the 
Australian Academy of Science,

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00:05:55,056 --> 00:05:58,183
which promotes and supports 
excellence in science.

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00:05:58,798 --> 00:06:03,154
Learn more about this topic 
and others like it at nova.org.au.

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00:06:03,207 --> 00:06:06,599
It was a blast of work with them 
so go check out their site.

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00:06:06,653 --> 00:06:10,540
Our videos are also made possible
by your support on patreon.com,

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00:06:10,689 --> 00:06:15,269
if you want to support us and become part
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00:06:15,269 --> 00:06:17,517
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