- [Instructor] The
planets, moons, and stars
make up less than 5% of all
the mass in the universe.
The rest lies in the realm
of absolute darkness.
The dark universe is the invisible,
yet dominating, component of the cosmos.
It includes a substance called dark matter
and a force called dark energy.
While little is known
about either, each helps us
understand the universe
beyond what the eye can see.
Dark matter makes up about
25% of mass in the universe,
but its composition is still unknown.
The substance neither
emits nor reflects light
and because of that, it cannot be seen.
The existence of dark
matter can be inferred
based on its gravitational
pull on the visible universe.
According to some scientists,
this force is what prevents
stars within galaxies from drifting away.
Dark matter may have also played a role
in the creation of the cosmos.
After the Big Bang, dark
matter formed a network
of filaments across the
universe and, in this network,
the first stars and galaxies were born.
Not much is known about dark matter,
but even less is known about dark energy.
Dark energy is a mysterious force
that fills the space between objects.
It makes up about 70%
of mass in the universe,
plus it has a repulsive property
that counteracts the force of gravity.
The reason for dark energy's
existence is unclear.
One theory states that it's
the fifth force of nature.
Yet another says it may help explain
why the universe is expanding.
In his Theory of General Relativity,
Albert Einstein considered a
repulsive force, similar to
dark energy, that pushed
space and particles apart.
Called the cosmological constant,
the variable may provide
clues for the expansion of space and time.
Much about the dark universe
is left to be discovered.
But with every theory and
technological development,
secrets of the invisible cosmos
will come closer to being revealed.
